The truth is that male dogs are better pets if they are neutered. They have less desire to roam, to mark territory, or to exert dominance over family members. Females are also better pets if they do not experience oestrus twice each year. Heat cycles bring hormonal changes that can lead to personality changes. Repeated heat cyles subject the reproductive system to uterine and mammary cancers and uterine infections.
Some bitches experience false pregnancies that can be a bother to deal with.
Dogs and bitches do not get fat simply as a result of sterilization surgery. They gain weight if they eat too much and exercise too little. Weight gain that seems to follow spay or neuter surgery is most likely a result of continuing to feed a high energy diet to a dog that is reducing his need for energy as he reaches his adult size. Excess energy in the food becomes excess fat on the body. As far as we know, dogs do not lament their lost capability to reproduce.