The title said it, but if it's not clear yet for you, maybe this is the exmple:
I'm a shy boy, well, i don't do many activities, i usually don't talk to someone i don't know very well, and very stiff at communicating......well, that's if i'm alone.
but, if i go for a walk with someone (can be my family or my friend) i think i changed, i talk more, have more courage, and i communicate easily with many people, and even sometimes i do the things that i've never done or i can't do before!. i think that's not me at all.
Well, do you think a presence of someone affect your attitude?