Buddhists who believe in re-incarnation think that your deeds in this life, especially the bad ones, but also your wishes keep you on earth. They are the karma that you bring with you when you are re-born.
When you finally manage to let lose all your wishes and hurt nobody (which also includes animals ) you are ready to enter the nirvana and the wheel of re-incarnations will stand still.
So it is a very impressive idea of humanity trying to get better with every life. While I do admire these ideas I find it difficult with my cultural background to really believe in them.