Well, the most fundamental difference is that Dana (and then eventually Kevin) travels through time. This means that her interaction with each of the men is interrupted by markedly non-standard means. (In other words, she vanishes.) This means that they wonder about her, and that she skips forward in their history, understanding things with new perspective. Simply being from another time provides another fundamental difference. She must see things differently, because she is not of that time. On a more symbolic level, they are all bound together, by fate, in ways that other people are not. That means their interactions are more meaningful.