I wonder if that's more defined as unconditional love than altruism. If you are actively loving her and not just having the feelings of love, that sounds like you're doing SOMEthing to make her life better, which gives you the good feels.
'tis quite a puzzler for me. A lot of times I will do "good deeds" and question myself as to WHY I am doing that. What do I want (now or in the future) in return? Or is it just because I want to feel good about myself for doing something nice... a tick in the "getting into heaven" column, if one believed in that sort of thing?
I've ventured down this road as far as I wish.