I've been there, I've done that. I have stayed at jobs long after I should have moved on because the people I worked with were great and I got a lot of social interaction from fellow employees. It wasn't as if we hung out together after work or on weeekends but they really were the ones that made going to work worthwhile. Unfortunately, they were also what kept me from new challenges and experiences.
Why was I afraid to move? Like most people, I knew what I knew and didn't know what I didn't know. The familiarity of the job (you can substitute "unchallenging"), the familiarity of routine (I knew how long it took to get to work and how long to get home), and the familiarity of co-workers were all the factors that kept my comfort level high. Well, at least it's a job, I would tell myself.
Change is scary, but I have learned to close my eyes and jump at new challenges. As someone told me recently, if you are an engineer and you get something wrong, people could die. If you are a graphic designer and get it wrong, no one dies. That certainly puts things in perspective. It also provides an incentive for me to try new things and not be afraid of failure. What is the worst that can happen?
Here is a challenge for you: if you take the leap to a new challenge, a new learning experience or even a new job, what is the worst that can happen? Now turn it around and think what is the best that can happen? Instead of dreading Mondays, look at the day as a fresh start; a fresh opportunity to try something new and to step out of your comfort zone. Remember, too, that blue is actually a beautiful colour. It is the sky, it is the water, it is fresh and new. Turn it around and see what happens.