I think what causes one to love is entirely dependent on each individual. Not only does the definition of love change from one individual to the next, but the reasons themselves are also just as variable. One might not know why they love, one might understand perfectly why they love someone. One might love for money, one might love for safety and security, one might love because they’re lonely.
Chemicals are involved, yes… But even those do not describe love in its entirety – because we all experience something different. Love is as real as people want it to be or need it to be. Love is just as real as hate or any other emotion.
Edit: Love can also be viewed as a survival mechanism. People are far more likely to survive and succeed if they have one or more people by their side. Not a single person can get through life alone, at some point in their life or another. With companions willing to help you, stand by you, etc… Life becomes much easier, in many regards.