I think I remember an earlier thread — or it may have actually been a web-page I stumbled across elsewhere — that talked about English set phrases such as this, where two words which appear to mean the same thing are used together. It suggested that this was to do with the origins of modern English; one word would be of Anglo-Saxon (Germanic) origin while the other was of Norman (French) origin. I think it was something to do with documents needing to use both words so that all sectors of the community would understand what was being said.
This definitely looks like an example of that concept to me, love being of Germanic origin and affection from the French.
EDIT: Scrap the "definitely"; this is a case of my fingers typing faster than my brain is working (not unusual!) I think Harry Batt is right on this one, and it's not one of those Anglo-Saxon/Normon combinations.
Used together, the terms "love and affection" are idiomatic. Though they seem to have the same meaning, there is a difference depending upon the context. It would also depend on the nature of the person who is the speaker. A teenager is more likely to "be in love" because that is what his or her friends talk about. Affection, which is the nature of kissing, hugging, saying nice things about the listener or doing nice things for the listener is not the term he or she would use.
With pets (and people) it is possible to show lots of affection absent the love aspect. You can pet an hug a person or dog without feeling any real emotion. It can be used as a "tool" to get what you want from the dog or person.
For example, a professional trainer will give a dog abundant praise and copious hugs when the dog performs as requested. This may or may not mean that the trainer loves that dog (or dogs in general). It is a tool for training.
It is presumed that a Guide Dog (or a Seeing Eye Dog) will be both loved and showered with affection at home. There are some very reserved people who love their pets and do not show much affection.
Note: Guide Dogs [for The Blind] and Seeing Eye Dogs are properly capitalized if they refer back to the organization that trained them. They can be lower case if they are being used generically. It is like "scotch tape" (generic) and "Scotch Tape" (brand name).