Is it dangerous to have scar tissue under your kneecap?

I've had knee problems for 3 years. There was no injury, and XRays, MRIs, and CT scans show no tears in the joint or surrounding ligaments/muscles. From what I've been told, the muscles on the medial part of my kneecap are very tight and do not allow my kneecap to move correctly. I have never had surgery. One of the symptoms that I have is crunching and catching in my kneecap. My physical therapist told me that this was because of scar tissue in the joint. Is it dangerous to have scar tissue under my kneecap? And how is it possible that I have scar tissue when i had never had an injury, nor have I ever had surgery?