Why has everyone kissed each other?

mainly everyone has kissed each other.

Chandler kissed Monica when they were going out obviously, he kissed phoebe when Phoebe and Rachel found out about Monica and Chandler, He kissed Rachel when she was visiting their college.

Joey kissed Phoebe for her birthday, he kissed rachel while they were going out, i don’t think he kissed monica though

Ross kissed rachel while they were going out , he kissed monica while she was lying down in the bed after chandler kissed rachel, and he kissed phoebe after he found out about carol