What makes a person fall in love is when the mate hones in on a need or want that had been missing in your life. Some will state that it's a good character, sex, chemistry, friendship, whatever that provides that initial spark. In the case of some of my boyfriends, it was their characters and personal charm that appealed to me.
However, I once inadvertently fell head over heels over a man (not an appropriate situation at the time so the relationship never became sexual). We had nothing in common in the sense of activities but he was a decent sort. He was athletic and intelligent. But the qualities that made me fall in love with him was the empathy, kindness and loyalty he showered on me. We never did engage in a romantic relationship but we were attracted to one another.
I won't go into any further detail about this matter other than to state that I was amazed that it was kindness and empathy that made me fall in love with him. Now I understand that this occurred because these were qualities that people had rarely shown me. I grew up in an abusive household and, well, sometimes the world itself can be a cruel place. That simple quality of kindness of his broke my heart in two and I'll never forget the compassion he showed me even though many years have passed since we last saw each other.
In essence, what sparks love is when the other partner, either purposefully or inadvertently, hones in on a quality or feeling in your life that you have lacked or not experienced before. This person may not make the best husband or boyfriend but if he can hone in on what is missing in your life, you're hooked.