Some reasons why animals might be scared is because some of the shelters might scare them because they are being taken away from their owners which is bad and can lead to depression for the animal which will make them really distant from other animals and from other humans. Dogs can also be really scared of humans if the animal has been abused from their pet owner which might make the animal scared of all people and the animal might not trust anyone. If you are trying adopt a pet and the animal might look scared or shy don't just ignore them try to play with them or maybe even adopt it so thee animal won't be scared anymore.
Make sure you have all the things you need to take care of a dog like a bed etc. Make sure that your pet has all the shots it needs before it leaves the shelter and you take it home. Make sure that your house is right for the type of pet you have if you live in a apartment, getting a big dog would not be a good idea. If you have a small child a puppy would not be a good choice.
A good connection between the owner is important it's good because if you and your pet have a bad connection the dog might end up sad because you aren't playing with it or not connecting with it. If you go to the shelter and you play with the dogs and you see that you don't have a connection with the dog don't worry just move on to the next animal you might have a connection with. Dont adopt a dog you dont have a connection with. Part of being a pet owner is having a connection with aniaml.