What you may be experiencing is just a basic rapport with a person. It can be for any number of reasons. Chemistry being one. Literally. Pheromones. They might be attractive to you, and gives you a sense of connection, or a desire to be around that person, regardless of gender or sexual orientation.

The other one is rapport. Whether this person talks to your or not, even something as simple as them moving similarly to you can cause a subconscious rapport between the two of you which will result in a similar feeling in one or the both of you (depending on how sensitive your minds are to things like that.