You talk like you owe your boss a favour. So you want to perform well and be loyal to him or in other word slave.  Pls, there is no who owe who. You contribute and the boss pay you. That is the relationship between you and your boss. If you can find others better company, go ahead give him the ultimate letter. In working world, people understand your situation.

Last but not least, you decided what your future going to be, not your boss. If one day you are in deep trouble, will your boss help you. Answer is no. You are on your own. So stop showing your loyalty and appreciation to your boss.