I'll give an example of the rebound relationship I was in. Even though we were together for 8 months she still left me for her ex because we got together shortly after they broke up. Her reason was she literally said she still loved him. I actually still missed for a while after she did that hurtful shit lol.
Bottom line when you see someone so often you are close too and they just are gone, it takes time to really adjust to that. Its the only reason i think the author is causing the actions of the characters to be more realistic and complicated