A dog is an animal that sometimes isn't treated very well. Dogs like to play outside and roll around on the ground. They also like smells that people might not find very pleasant. If a dog has been outside rolling around in something smelly, it will come back smelling very bad. People will avoid it, wrinkle their noses and probably treat it very poorly. They don't want it around, and they don't want to touch it because it might make them smell bad. If a person is being treated this way, the person is unwanted or scorned; other people find them disgusting."back to being treated like a dog that had rolled in something smelly"
I'm guessing this is in the context of someone's birthday being ignored. Most people on their birthdays will get a nice gift and a cake from their family and friends along with a party, cards and well-wishes. This person has never received a cake or an appropriate (real/nice) gift, but he still thinks it's strange for people to ignore the event. They should at least say something to him about his birthday."they'd never given him a proper present, let alone a cake-but to ignore it completely.."
Someone is offering to take a guest (or other person) through the rooms or hallways of the house to the dining room, a room of the house designed for eating a nice dinner. It is the sort of thing a butler might say."May I take you through to the dining room?"