Why you shouldn't go to failure every set?
It's unrealistic for a novice lifter to train to failure on every set if you're going after muscle development. ... If you can't hit the same number of reps, it reflects less on your strength and more on your muscles' conditioning and current level fatigue. In many cases, that's a good sign.
How many sets should you do to failure?
Increasing the number of sets taken to failure from one set to two, three or four provides no more benefit than doing just one set to failure. In fact, taking more than one set to failure may actually blunt strength gains. Take-home message: for strength, do no more than one set to failure per exercise.