He enjoys the challenge
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.
Harry is "special"? (Speculation)
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.
Wand-crafting and sales is at the confluence of art and technical skill
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.
Process of elimination
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.
Olivander has a deep reverence for wand-pairing
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.