I would need to make the distinction between romantic love and unconditional love, because I think different biological factors are involved. In short, my experience is that romantic love can have a "shelf-life", whereas unconditional love is a bond for life and for evolution (i.e. the survival of the species is dependent on the mother child bond).
It appears you are referring to romantic love only, so I'll try to lend my opinion on this. There are evolutionary factors involved here too, but I think romantic love is a process where we take all of our previous experience and project it on another. In other words, when we look for a partner we use what we have accumulated, in terms of an "ideal mate," based our experiences throughout life. Those who fit, or at least come the closest, are the ones we are attracted to. And since this person we've chosen "fits our ideal" we in turn open ourselves up to reactions in the brain (again, evolutionary) that send a cascade of chemicals - namely dopamine, which is a neurotransmitter that controls risk/reward and pleasure.
So basically, I consider "falling in love" to be just a chemical reaction based on an archetype we've accumulated throughout life. Hence, when we grow and change over the years we also change our perspective (archetype) and thus we might then "fall out of love" with the person who used to fit our ideal - the 7 year itch.
To me, spirituality, if it exist, comes in to play with unconditional love only, since that seems to be the kind of love that's transcendent.
wf