If you are with your crush and you don't have plans on courting her then you won't feel very nervous. But I you are planning to court her and you are with her then that is a serious problem.. You'll be nervous because you are always thinking on how to tell her, what to tell her, is it the best time to tell her, is this the best place to tell her and so on.. Asking all of those questions and being not to sure of the outcome makes you nervous because You're in a cold place (say in a Mall) and you're sweating like you've just had a shower.