So my friend had a cat. She went on vacation for 3 weeks. She had someone check on the cat, but before she got back it died. She found it in a closet and it was dead. The person who checked on it came twice a day. So did it kill itself because it was depressed because it didn't receive a lot of attention?

She dies of hunger.

the person that checked on it might of forgot to feed it or give it water whether they said they did or not, or it might have been sick or just gotten to an old age

Was the friend actually checking on the cat???

the cat might of been locked in the closet or the person checking on it didnt look for the cat and just left the food and water but didnt bother looking for it.

You're kidding right?
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Was the cat old? It could have just died of old age. But as far as I know, yes, animals can die from loneliness, but that usually only happens after they lose a mate. They just give up on life. I guess it's possible that your cat had thought it had been abandoned and gave up. This may be a stupid statement, but, the checker did remember to give the cat food AND water, right?

i would ask the friend that watched the cat but it could of died of lack of food or water.