I have a friend I'll call Holly. She sees the best in people and consequently the best in people shows up. I've seen her bring out the best in people for whom the best is rarely seen. It's like magic.
If you are rolling your eyes before you even talk to that person, then you are dead in the water.
Dr. Walter Brackelmanns says that we are supposed to see and treat others as if they are competent and mentally well, even if they are not. Our beliefs, expectations and assumptions matter and muddy the interaction. Consciously or unconsciously, the other person "knows" what you are thinking and feeling. Even if you smile, your assumptions will seep or shine through in the conversation.
Whatever we focus on grows. Whatever we believe, happens. Thoughts become things.
Feel free to experiment with this concept if you are feeling cynical (no wonder the worst in others comes through). ;D
Endeavor to find the best in someone (it's there, guaranteed) even if you have to become a super sleuth detective to find it. Watch what happens - it's like magic.