First: Daisy is an adorable, 17-pound Mastiff puppy. What you have to ask yourself is will there be room for both of you in bed when she's a full-grown, 130 pound dog? You have to remember that her legs don't bend the way ours do - picture her, as an adult, taking the space of a four-foot square. Quite possibly not the best choice for a sleeping companion in your twin bed.
Next: Does your pup snore? Or shed? Or drool? Much as you may love him, there will be times when he's been rolling in something that you really dont want on your sheets. Giving a dog a bath every night before bed-time isn't very realistic.
Third: A dog's normal body temperature is somewhere around 102 degrees. This can be warm, wonderful and cozy on a frigid winter night - especially when you're trying to keep that thermostat set low and the energy bills under control. It actually sounds delightfully snuggly this time of year. But think about those same 102 degrees in the hot month of August. Will you really be able to look at that shaggy face and say "Not tonight, dear, it's too hot !" Your dog isn't going to understand and may think he's done something wrong.
There are some trainers who claim that dogs should never sleep with you, that it will undermine your authority as the leader of your family "pack." The most important concepts in training are patience, decisiveness, and consistency. Decide what the rules are for your dog, and stick to them. As long as she knows what's expected, she's happy.