Barry and Caitlin have been friends for quite some time. In high school Barry starts to admire Caitlin. Does she admire him. Both Barry and Caitlin are going through some trouble with there powers. But love can defeat all.
Based on the recent events which have occurred, like Barry Allen seeing in the speed force, Caitlin and Ronnie's baby and wanting to be his. Moreover the fact he broke up with Iris. Barry thought maybe Iris wasn't the right person for him and maybe someone else was...
All these events brought Barry and Caitlin closer to each other and that's what kick-started their love story...