A tour through the average American animal shelter inevitably breaks the heart. So many dogs and cats peer out of kennels and cages with expressions of hope, it's hard to walk away, or to choose only one.

Sadly, many good-intentioned people adopt pets without realizing the amount of responsibility that goes along with it, and without knowing how to cope with the inevitable adjustments that a pet requires.

In essence, a lack of education is the reason why millions of pets are given up to shelters each year. Better education and preparation, done before a pet is even adopted, can head off problems that so often lead to relinquishing pets to shelters.

But specifically, what are the reasons why so many animals are given up each year? A series of studies conducted by the National Council on Pet Population Study and Policy sheds light on this question.

Some of the reasons are unique to the type of pet – dog or cat – and some overlap. Here are the top 10 reasons why people give up on their pets:

Dogs

1. Moving
2. Landlord issues
3. Cost of pet care
4. Lack of time
5. Inadequate facilities (such as no backyard)
6. Too many pets in the house
7. Pet illness
8. Personal problems (such as sickness in family)
9. Biting
10. No homes for littermates

Cats

1. Too many cats in the home
2. Allergies
3. Moving
4. Cost of pet care
5. Landlord issues
6. No home for littermates
7. House soiling
8. Personal problems
9. Pet illness
10. Inadequate facilities

Some of these reasons are related to others. For instance, moving is a common reason for giving up a pet, but it often involves landlord issues (not being allowed to have a dog in an apartment), housing issues (small or no yard) or that other people in the home would not permit animals. But the studies also noted that more than half of the respondents noted some behavioral problems as well.

People who tended to give up their pets were usually younger, moved more often and owned the relinquished pet in question for a much shorter period than those pets still in the homes. The risk of surrendering a pet tended to decrease as time passed. Other findings included:

Dogs and cats that were not sterilized were at higher risk of being given up

Mixed-breed dogs were given up more often

Dogs from a shelter, friend or pet shop were at higher risk of being given up

Men were more likely to give up dogs and cats

Outdoor dog and cats were more likely to be given up

The studies also pointed out that dogs trained to be obedient and had regular veterinary care were more likely to be kept in the home. Cats obtained as a stray also were more likely to bond with the owner and remain in the home.