It might be caused by some hormonal processes, a good and experienced doctor should be able to help. Meanwhile, a suitable lubricant can help, especially if you don´t have any allergic or other negative reactions to it. I agree that it is quite a common problem.
Are you on birth control? A lot of times your lubrication can be affected by birth control pills. You might ask your doctor to switch the kind you are on and see if it improves. That happened to me and was a constant problem. I got an IUD and never had to worry about it anymore. For now...invest in some lubrication and be sure to let your partner know that it's not because of him. It's a very common thing. I wouldn't recommend the flavored lubrications though because they can cause infections.