From what I remember Kes grew up at her underground colony and then her adventurous spirit caused her to go up on the planet's surface. There she was captured by the Kazon to be a slave. I believe she and Neelix met maybe once or twice, Neelix wanted to save her (which he did with Voyager's help) and then came on board.
I believe their relationship was completely platonic but they did love each other. Like Kes said in one of the episodes, or one of the books not sure which one, Ocampas mate for life and there is no jealousy. They pick a partner and that's it. But aboard Voyager, Kes broke free of the restraints Ocampas have. She had a career, well two of them actually, she had friends, her gifts grew and suddenly an Ocampan life just didn't cut it anymore.
As for Neelix, he found his own rhythm on the ship as well. His problem was that he still saw Kes as the abused girl that needed to be rescued. Only this time it wasn't from the Kazon but from the sexual advances of the various crewmen. While his jealousy seemed irrational he was right on the money when it came to Tom. Tom did have a crush on Kes and even saved his rations for a while to buy her beautiful pendant.
In any event, I tend to see them as a May/December romance. Pretty good while it lasts, but it's simply not meant to last. In the end, Neelix was tired of traveling, wanted to settle down and found a woman of his own species. Someone who viewed him like a hero and who had the same ideals and wants of life as he did, while Kes also moved on.