This question is a bit ambiguous, because “Callahan” and “Mike” could refer to several different people or characters in different stories. Without more context, the most likely reference is to Merle Callahan and Mike McLusky from the TV series Mayor of Kingstown.
Likely TV context
In Mayor of Kingstown , Merle Callahan hates Mike mainly because he sees Mike as a traitor who turned his back on the white‑supremacist prison gang Callahan belongs to, and then used his influence on the outside to work with rival groups and against Callahan’s interests. Their conflict is also personal: Callahan believes Mike tried to have him killed and then used the prison system to box him in, so Callahan targets Mike’s family (especially his brother Kyle) as revenge.
If you meant someone else
If you had a different Callahan and Mike in mind (for example, real people you know, or characters from another show, book, or game), please specify which story, series, or context they are from. With the exact context (title of the show/book, or who they are in real life), the reasons behind the hatred can be explained more precisely.
