Why do I still stutter when talking alone?

Why do I still stutter when talking alone? My stutter tends to be worse when I'm talking to someone, but I've also noticed that I tend to stutter every now and then when I'm alone and talking to myself or reading something out loud to myself. What's the cause behind it? It's been driving me mad for years. I have a severe underbite (working on getting it fixed) that affects how I pronounce my L's, so I tend to trip up on words that involve the letter L. However, I still trip up on all kinds of words.