Why do dogs wag their tail?

Dogs wag their tails to communicate. A dog that's wagging it's tail isn't always happy... Watching other body language clues, such as body position, head position, ear placement, eyes can tell you if the dog is happy or there's something else going on. With a dominant dog, the tail may wag, but it is held high and just the tip may wag. A playful dog will hold it's tail high, but it will be wagging fully and quickly from side to side. A dog who's tail is held low and wagging stiffly may be submissive or fearful. Tails are a good indicator of emotion, when you look at the whole picture (body language).

For some comparision with people, dogs wagging their tails are like smiles to us humans, this is a sign of communication telling us that they are happy on what they are doing or what is happening around them.

It meens they are showing there happyness and yes they get excited too