In every part of your life, you are trying to move yourself into a mode of resolution. You've completed the cycle, and you'll do it for the rest of your life. This is what music does. The conflict in the predominant begs to be moved onto the dominant, which can then be resolved back to what it originally was. It's what drives you and the music on.
Dissonance is achieved when the use of thirds is interrupted.
This is what can make a phrase exciting. The dissonances available are minor 2nds, major seconds, perfect 4ths, augmented 4ths, minor 7ths and major 7ths. Now, if you start using dissonances all over the joint, your music will lack focus, goal, and will most likely sound like a person gargling on their own blood, or worse, someone else's blood. You have to really understand the construction of the classical phrase in order to understand how to use dissonance effectively. Let's look at a song that most of us will probably know.