I will make a few assumptions here:
- We're talking about the Westerosi only
- Only nobles, no smallfolk
- They married when they were in love, unlike Ned and Cat who found love after they were married
- The love has to be mutual (sorry Jorah)
Tywin and Joanna
It is ironic that the hardest man in Westeros, one who feels nothing is more important than solidifying his family's heritage and rank; marries for love, and his own cousin no less. This is what makes their marriage a loving one, there was no advancement, not pact, no alliance to be made; just love.
Tyrion tells us this:
Lord Tywin seldom spoke of his wife, but Tyrion had heard his uncles talk of the love between them. In those days, his father had been Aerys’s Hand, and many people said that Lord Tywin Lannister ruled the Seven Kingdoms, but Lady Joanna ruled Lord Tywin.
-A Song of Ice and Fire: Book 3 - A Storm of Swords, Chapter Thrity-Eight (Tyrion V).
Most say that Tywin lost his smile after Lady Joanna died.
Tyrion Lannister and Tysha
Don't make me cry. Leave me I don't want to talk about it.
Where do whores go?
Targaryens aren't loveless
Aegon V Targaryen and Betha Blackwood
Our old mate Egg, lived with the smallfolk, started from the bottom and fell in love. It should be noted that he was able to do this mostly because he was so far down the line of succession that it wasn't politically damaging for him to do so. However, him and Betha loved each other.
This is funny because due to their actions their children (as pointed out below) also married for love following Aegon and Betha's example.
Prince Duncan Targaryen and Jenny of Oldstones
They eloped. Prince Duncan - also known as Duncan The Small and Prince of Dragonflies - was the first son of the King, King Aegon V Targaryen (Egg) and was betrothed to a daughter of Lord Lyonel Baratheon (The Laughing Storm), but Prince Duncan broke the betrothal and eloped with Jenny of Oldstones, giving up his right as heir to The Throne.
Prince Jaehaerys II Targaryen and Princess Shaera Targaryen
Prince Duncan's second brother and Aerys "The Mad King"'s father. He too was betrothed (to Celia Tully) but he loved his sister Princess Shaera Targaryen, Aegon's eldest daughter (who was also betrothed - to Luthor Tyrell). They decided to follow in Duncan's footsteps and eloped and got married.
It was said of them:
from a young age, Jaehaerys had been of a more traditional frame of mind, as he was in love with his sister, Princess Shaera. Shaera, in turn, desired him. King Aegon V and Queen Betha observed this and were alerted because of it. They did everything they could to separate the two siblings, yet this only served to inflame their passion
Aegon I Targaryen and Visenya Targaryen and Rhaenys Targaryen
Yes, he married both of his sisters, his older sister Visenya and his younger sister Rhaenys, and they both loved him very much as he did them.
They didn't call him "Aegon The Conqueror" for nothing... (sorry).
Spoilers re Rhaegar:
Prince Rhaegar Targaryen and Lady Lyanna Stark
Although it is not confirmed, many believe that Rhaegar and Lyanna actually got married. There is a fair amount of evidence supporting this. If it is true, then it would mean that they were married for love.
Prince Doran Martell and Lady Mellario of Norvos
It's true, they're estranged now, but they were married for love (or at least they thought it was love). You see, they were attracted to each other, they had found something they thought was love; which we now know as infatuation.
The author, George R. R. Martin says this of their marriage:
It wasn't a good marriage. They married because of an attraction to something new and exotic. Sometimes, attraction happens when you least expect it. He was a prince of a distant country, and she was a woman full of life, who was very appealing, who came from a very different culture. When she comes to Dorne, she finds that there are customs that are different from those of Norvos, especially regarding the fostering away of children to others. This wasn't a political marriage, nor a magical one, it was simply an example of human nature. Sometimes relationships start on a good foot: you become acquainted, there's a great sexual attraction, you establish a relationship, you marry... and then in four or five years you realize that you don't really have anything in common, that at best you've made a mistake and are in a situation that doesn't have any easy solution in a society such as that of the Seven Kingdoms, where divorce simply isn't common. This is an example that it's not only marriages of convenience that fail, but even the marriages for love can fail.
Sometimes the marriages of convenience in the Seven Kingdoms come out well and those that are for love don't...
Davos Seaworth and Marya Seaworth
This is a bit "iffy". For one, they weren't nobles at the time they got married (he was a smuggler and she was the daughter of a carpenter). There's also no hard evidence that they love each other, but they do seem happy.
Edmure Tully and Roslin Frey
Another "iffy" one. They do definitely care for each other, and I'd dare say Edmure was smitten as soon as he saw her. She also seemed to grow fond of him, even during the wedding. She's is most definitely sad about the turn of events and wants to have her Edmure back.
Robb Stark and Jeyne Westerling
Another "iffy" one. I will include this one because it's entirely possible that even though Robb married Jeyne so as to preserve her "honour"; they could easily have loved each other anyway. They certainly seem to be at least caring, if not loving of each other after marriage, and they hadn't been married that long.