Pets lament their lost capability to reproduce. Pets do not to nurture their young for 18 years, watch them go off to college or whatever, marry, and produce grandchildren. Dogs and cats nurse their young for a few weeks, teach them to behave like dogs and cats, and go on with their lives. Males know next to nothing of fatherhood. The possibility of uterine or ovarian cancer is eliminated.
Neutered males have a lower chance of developing prostate infections.
They won't develop testicular cancers. Females will no longer go into heat, eliminating the probability of getting blood stains on your couch, floor, bed, etc. Both sexes experience less of a need for territorial marking behavior. Both sexes experience a decrease in the urge to roam. Both sexes become more docile and easier to train.