In my experience, there are a couple of different kinds of love (even within one person for another person). I'll use the two loves that I have for my husband as an example.
My love for him as a father/husband/lover/friend is conditional. I could fall 'out' of love with him if our values became so different that they caused us to be in conflict/anger/pain all the time. However, underneath that love is a second love... one that I have for him as a fellow human being - and this brotherly love is more compassionate, forgiving and kind than the first. The first kind of love can and does fail; the second kind not so much... and so it is this second love that is unconditional.
Tammy