I think there are many people who don't quite understand what love really is. Many mistake lust, or dependency for love. You can be in a relationship with a person, and both love each other dearly, or there may be no love present there at all. The worst type of relationship to be in, is one where only one of you is in love. Some people fall in love easily, and some struggle to see it at all.
For most, you can't quite understand what it is exactly, until you find it. Once you do find it though, you'll know exactly what it is, although in some cases there are those who aren't able to recognise it until they have lost it forever. It can present itself in various different forms for different people. Love is generally involuntary, but if embraced, and reciprocated, it can be extremely life altering, and blissful, but on the opposite extreme, it can also tear a person apart. I once read a quote, which compared love to a socially acceptable form of insanity. Essentially though, I beleive that love (true love) is a naturally occurring drive, which brings out the best in us, and causes us to care for a particular individual more than we care for ourselves.
'Love is always patient and kind. It is never jealous. Love is never boastful or conceited. It is never rude or selfish. It does not take offense and is not resentful. Love takes no pleasure in other people's sins, but delights in the truth. It is always ready to excuse, to trust, to hope, and to endure whatever comes.'
- I Corinthians 13
*Edit: Also, I beleive it is said that the opposite of love is not in fact hate, but indifference. If emotions seem confusing to predict, of course this is something us Aspies can struggle with (including myself at times). As you have stated that your style of thinking tends to be very logical though, this may be the answer to your question. Emotions tend not to be logical (though a study in psychology might assist to make it more so). They can occur despite our best intentions. If you'd like a scientific reason, I'm afraid you've already answerd it with "chemicals in the brain". A product of evolution, which enables us to select the genes which compliment our own. After all, if love were not to exist, would we bother to reproduce, and care for our young? Perhaps then this is more the answer you're hoping for:
BBC Science | Human Body & Mind | Science of Love