There are two ways to approach this answer.
The first way: Dogs are reacting in what looks like enthusiasm, but they are only delivering these actions because it serves their own purpose, not ours. They are domesticated, and rely on us for food and shelter. Over millennia, we have selected and bred only dogs that respond to us in a way that boosts our egos. To be fair, it's a win/win. We feel loved, and they get sustenance.
The other school of thought: Dogs are sincerely happy to see us. Dogs feel emotion. They worry, they care, they want to help, they genuinely respond, in their own sensitive doggy way, to outside events. They are a remarkable gift to humans (who are so unworthy), and they demonstrate a kind of beauty and sweetness that Mankind forgets is still out there.
My personal opinion? Well, I think that anyone who has been blessed enough to be loved by a dog knows the answer.