What is the source or cause of love?

More and more questions --

16. Why do we love? How does love (or being “in love”) happen? What causes love? What is the source of love? Where does it come from?
17. Is the cause or source of love something that is external to us, or is it internal?
18. Is love something that is necessarily temporary, or is permanent love possible?
19. What does it mean to “fall in love”? Is love really something that we “fall” into? And is it something that we fall out of?
20. Is love ready-made? Is it something that just happens or just doesn’t happen?
21. Is love something beyond our control? Is love the result of an uncontrollable force of attraction or affinity?
22. Is love thrust upon us, like Cupid’s arrow? Are we compelled to love without regard for our reason or free will, or even against our will?
23. Does loving someone depend entirely upon the other person? Do we love them because they “make us happy”?
24. Which comes first – attraction and happiness, or love? Does happiness and/or attraction cause or otherwise lead to love, or does love lead to happiness and attraction?
25. What is the source of happiness? of attraction? of desire?
26. Is it still love if it is painful or annoying? if we are disappointed or unhappy?
27. Do we love because the other person fulfills us and completes us?
28. Do we love the other person because of some attribute of that person – because they are physically attractive or smart or funny or honest or a good provider or someone that shares our values?