Harry tried. And tried. He had no idea what Mr Ollivander was waiting for. The pile of tried wands was mounting higher and higher on the spindly chair, but the more wands Mr Ollivander pulled from the shelves, the happier he seemed to become.
“Tricky customer, eh? Not to worry, we’ll find the perfect match here somewhere- I wonder, now – yes, why not – unusual combination - holly and phoenix feather, eleven inches, nice and supple.”
Why was Ollivander happy about pulling the wands?
It probably just means he loves his job.
There's no particular reason to think that a "tricky customer" who needs to try many wands is any better than any other. But Ollivander seems to be someone who's found his true vocation, and dedicated his life to it. Handling wands just seems to be enjoyable for him. Maybe he's remembering his process of making each individual wand - he seems to know them all personally - or maybe he simply likes the feeling of handling them one after another.
I'd say there isn't too much to be read into this phrase. It's just a throwaway line in the middle of the passage, probably not something with any big significance. It helps to build an impression of Ollivander for the reader, as a cheerful and competent old man who knows what he's doing and loves his job - no more than that.
Ollivander seems to be fascinated by things beyond the ordinary in magic. After Harry’s wand chooses him, Ollivander says:
“Yes, thirteen-and-a-half inches. Yew. Curious indeed how these things happen. The wand chooses the wizard, remember. . . . I think we must expect great things from you, Mr. Potter. . . . After all, He-Who-Must-Not-Be-Named did great things — terrible, yes, but great.”
This is emphasized again when Harry interviews Ollivander in Deathly Hallows:
“The owner of the Elder Wand must always fear attack,” said Ollivander, “but the idea of the Dark Lord in possession of the Deathstick is, I must admit . . . formidable.”
Harry was suddenly reminded of how he had been unsure, when they first met, of how much he liked Ollivander. Even now, having been tortured and imprisoned by Voldemort, the idea of the Dark wizard in possession of this wand seemed to enthrall him as much as it repulsed him.
Despite the terrible things Voldemort has done, including to Ollivander himself, Ollivander seems fascinated by Voldemort’s unique magic and how that would become even more pronounced with the Elder Wand. It’s almost as if he wants Voldemort to get the Elder Wand just to see what awesome magical feats would occur.
For such a person, the situation when Harry bought his wand must have been quite exciting. A wizard already suspected of being extraordinary cannot find a single standard wand to choose him. As each additional wand joins the pile it becomes more and more exciting, as it indicates that this is indeed a special case.
This was addressed (indirectly) by JKR in her writings on Pottermore. In short, Garrick Ollivander is a wand nerd, obsessed with finding the very best match for each of his customers. A tricky customer is therefore, by definition, much more interesting than a mundane customer.
Mr Ollivander, however, was a purist who insisted that the best wands would never be produced merely by encasing the whiskers of a favourite Kneazle (or the stalk of a Dittany plant that once saved a wizard’s father from poisoning, or the mane of a kelpie a witch had once met on holiday in Scotland) in the customer’s favourite wood. The best wands, he believed, had cores of immensely powerful magical substances, which were expertly enclosed in specially selected and complementary wandwoods, the result to be matched to an owner with whom the wand itself felt the most affinity.
and
The following notes on various wand woods should be regarded very much as a starting point, for this is the study of a lifetime, and I continue to learn with every wand I make and match.
Additionally, his reputation is on the line here. Harry is a VIP customer and getting his wand just right will be a big feather in his cap, amongst those that know about these things.
While there was initially substantial resistance to this revolutionary way of crafting wands, it swiftly became clear that Ollivander wands were infinitely superior to anything that had come before. His methods of locating wand woods and core substances, marrying them together and matching them to ideal owners are all jealously guarded secrets that were coveted by rival wandmakers.
He's a man who prides himself on his work. The more difficult the task is, the more it proves that this is something that requires his expertise. Presumably, someone of lesser skill could not even get that far.
Others have mentioned that he likes his job and has a fascination with magical power that can be channeled out with unique, hand-crafted wands.
We don't see him helping others in detail, but I would assert that, though he's passionate about matching each witch and wizard consumer with their apposite wand, respectively, there must also be some elevated savor associated with solving especially difficult match-making trials.
The most complex or complicated pairings free Olivander from an otherwise more tedious and predictable exercise (that's still fun but not ideally so). It also enables him to continue to grow at his vocation, even though he's already a master at it. This demonstrates that there's always still some potential for growth even for someone so skilled, seasoned, and advanced at a trade as he is. He's probably conscious of this.
Aside from being a complex case study, Harry also has prestige, though almost congenitally and incidentally so. Being the one to solve the "which is the ideal wand for Harry Potter?" question could possibly fill him with a kind of joy or pride, while also reaffirming his abilities to himself and enhancing his store's reputation. This part, of course, is just speculation.
Creating his own wands, he wants to perform the pairing process to the best of his abilities, too, in order to affirm that his wands go to the right witch or wizard. There is quite a lot of technical skills that go into composing or fabricating the wands, but then sorting them to the right owners becomes more of an "art" that requires a different kind of skill.
He knows that there's an "art" to this, and is happy to entertain the methodology.
He may be getting increasingly thrilled because he's factoring out the likely candidates that failed to partner with Harry.
He knows that eventually one of the wands will work, but until then he's in an excited state of animation. Maintaining the momentum and testing as many promising options is likely the best course of action when a pairing proves difficult.
The services that he provides have profound impacts on the next generation, so it's ideal that he demonstrates a plenitude of respect, pride, and fondness for and of his work.
