this other person feels physically attracted to them.
you behave playfully as if you were attracted to each other.
you are determined to make life difficult for them.
their relationship goes through good and bad patches.
you become friendly again after a disagreement.
you fall madly in love with them very quickly.
To have a romantic relationship without your partner knowing.
you start a conversation with them in a friendly way because you are attracted to them.
you are attracted to them in a romantic way.
you like them and become friends immediately.
flirt with someone
make up with someone
hit it off with someone
two-time someone
someone turns someone else on
fancy someone
fall head over heels for somebody
chat someone up
have ups and downs
have it in for somebody