Your observation is simple and correct. When people act like they like you, they act in a good mood around you (as though they are happy to be with you). This means they like you. When people act like they are not happy to see you, and not a bad mood for other reasons, then they probably don’t like you.

They may like other people better, but it’s not so healthy for you to compare yourself to others. Focus on just the relationship between you and the other person. If they act unhappy to see you, you can ask them why and if they have some issue with you. Maybe you can work out differences and build on the relationship.