There are always people who will be negative towards you. You must learn to accept this. There is no point in trying to win this people over unless there is a real great reward in doing so or real pain failing to do so. There are two types of negative people. There are those who show directly that they do not think of highly you and there are those who outwardly appear as your friend but are subtlely putting you down.
Avoid the first type. Beware of the second.