First of all everyone should know this answer. But in case you don't know sometimes someone as in your classmate can be allergic to any sort of animal. Sometime it just depends on the kind of pet. If it was to be a dog then there are quite a few reasons. This question is fairly easy to answer.
You would need to introduce a list.
For example you could have: My shopping list contains of; milk, bread, and yogurt.
Another purpose is to separate clauses.
It’s A. This river is WIDER than that river.
Well if something bad happened to him maybe its coming back more frequently and traumatized by it depending on what it is or if something had triggered those memories to found a way back,
