Are you an expert?..... In telling what cannot be done? Almost unfailingly when a new idea is presented? Do you exercise your "NO" muscle more than your "YES" muscle? I'm not suggesting you take on unrealistic things or agree to near impossible tasks to exercise "YES" muscle, but I'm frequently amazed how some people use all their creativity and try convincing why something cannot be done almost automatically. You might want to catch yourself when you are saying NO and instead replace with "YES, but". The problem with NO is that people will brand you as stubborn/hard-to-work-with and soon no opportunities will come your way.
Richard Branson said "If somebody offers you an amazing opportunity but you are not sure you can do it, say yes – then learn how to do it later!"