Here is an article from a cat rescue forum explaining how a cat's personality changes when it is neutered.
The Benefits of Spaying or Neutering Your Cat
Each year millions of healthy and lovable cats and kittens are euthanized at shelters and pounds in our country because their numbers greatly exceed the number of available homes. While some pet owners claim that they have no trouble giving away their kittens, they should realize that their babies take homes away from other unwanted kittens. Also, if they were to call the adoptive homes one year after the adoption, they might be surprised to find that very few kittens still lived with the families that adopted them. Overpopulation devalues the quality of life for all pets.
Many cats are surrendered to humane societies or abandoned at the age of 5-6 months. At this age they have outgrown their kitten cuteness and are beginning to exhibit adult behaviors such as territorial marking. While spaying (females) and neutering (males) can not make them remain eternally kittens (at least size-wise), sterilization can eliminate a number of annoying behaviors, such as:
The howling, pacing, and housesoiling of a cat in heat. (A heat can last up to 21 days and can occur three or more times a year). The caterwauling of toms looking for romance outside the house is also eliminated.
The urge to roam to find a mate which may involve travelling long distances, getting into cat fights, upsetting neighbors, all of which may result in possible injury or death.
Urine-marking. The intact male's urine is especially foul-smelling. Both sexes can engage in territorial spraying.
An altered cat is likely to be a calmer, healthier, more content pet. Studies show that spayed and neutered cats have average life expectancies twice as long as those of unspayed and unneutered cats.