Life is very messy, in general, and does not follow neatly organized lines. Feelings are especially messy and non-linear. For this reason, boys fall in love with girls who are "attached" to someone else every day. At any age. Maybe more at OP's age than later because people, perhaps, become more rational and cautious with age, but I would not make this claim without having data at hand.

The same is true if you reverse the genders: girls fall in love with boys attached to someone else every day. C'est la vie. To expect anything else is counterfactual.