When Becky and Sara meet for the first time, Becky is in a bad mood. Perhaps she's snappier than usual and makes a rude remark either to Sara or someone nearby. Maybe she completely ignores Sara. Whatever the situation, Sara decided that Beck rubbed her the wrong way and she doesn't like her at all. This 'first impression' can taint all their future interactions.

Mark has heard from many friends and acquaintances that Fred is 'bad news'. Some say he's manipulative, creepy, shallow, egotistical, etc. When Mark and Fred finally meet face-to-face, Mark already has preconceived ideas about Fred and expects the worst from him.

I'm sure you've heard the expression "guilty by association". Let's say, Diane doesn't get along with Maggie for whatever reason. By default, Diane distrusts anyone that is friendly with Maggie.

Even if the characters are on the same side, they may have different ideas/opinions that they are unwilling to budge on.

Misunderstandings often begin as something small and can escalate out of hand quickly. Let's say Carrie hears only a small part of a conversation and draws her own conclusions without verifying anything with her friend/coworker. Carrie gets the idea that she was wronged when in reality the conversation had nothing to do with her.

Jealousy is a vicious force whether it's something physical, emotional, or something else. Beth may be jealous of Linda because of a relationship, opportunity, etc.