Why do some people think that they are better than others?

Unfortunately, it's an age old dilemma and has been the cause of persecution, oppression, slavery, disputes and wars throughout the centuries. Even a younger America, with it's declaration that all men were created equal, allowed slavery and segregation to exist (until it finally came to its senses, for the most part). But pride and prejudice still continue and those that are viewed as inferior still suffer. I personally can't your question as to why people think they are better than others, but I can offer some Biblical support on the effects of pride:
Read Psalm 73, Proverbs 11:2, Proverbs 13:10, Proverbs 16:18, Proverbs 29:23, Isaiah 2:17, Psalm 10, 2 Chronicles 32