Many players judge the success of their serves by how many aces they hit. Quite often, these players negate their aces by a tremendous number of double-faults. Many players judge their serve by minimizing the number of double-faults. These players are happy just to get the serve in, thus, they are defensive on their serve and seldom win any points directly as a result of their serve.
A much better indication of a good serve is how many points you average winning in your service game without your opponent putting the ball in play. If you can average winning one or two points per game as a server, it takes a lot of pressure off you. When the match pressures increase or the heat is a factor, this is very important. You cannot physically continue playing 50 ball rallies. Try thinking about your serve this way.
Good luck on the court!