I don't know if this is something that I can logically define and understand like I do everything else;
I've been dating this girl (ISFP) and I think I'm in love, but I don't know. Most people, when they define love it's feelings based (the sun shines, the birds sing, that uncontrollable smile, etc etc etc). I'm not and have never reacted that way to anything. I do care about her, and despite some of the mental battles I go through when making time to be with her (sometimes I just don't feel like it), we end up having a good time together. She doesn't require too much touchy feeling interaction (although I feel she's holding back and would like more than I give- she says she's fine).
I go from really digging her, to being annoyed, to feeling comfort, to wondering if she's 'the one'. From what I read here, I'm not alone in my quest to analyze everything to death, but with love it seems like you can't analyze it all, because another person's involved and you don't know and won't know it all about that person in a reasonable amount of time, before they'll be fed up and leave you LOL.
A good friend of mine - don't know his MBTI, but he said he wasn't one of those guys who is head over heels in love, but rationalized that if it's worth more to be with her than without her, then he had his answer. They've been married for about 5 years, and 2 kids, and he's happy.
So my fellow ISTJs, and the Ns that will come in even though you aren't 'officially' invited (Ns run amuck in these forums), how do you know you are in love?