Dogs take work and commitment:
A dog is a living animal and requires all the love and care that you would give to a child. When you take on a dog there is a responsibility that goes with it. Dogs do not look after themselves.
They may be expensive - there is a range of costs that they can incur - feeding, vet bills - regular inoculations, general health and any accidents/illnesses that might befall your dog, someone to stay with them when you are away.
They need time - walking (don't expect dogs to take themselves for a walk), feeding, playing, training, grooming, and cleaning up after them!
They need company - so it is important that you are able to spend time with them during the day. Dogs are pack animals and they look to you to be the leader of the pack. If you do not give them enough attention (walking, training, grooming, feeding) they may try to take over as pack leader and become aggressive and uncontrollable.