A lot of people find it difficult to accept the fact when they get diagnosed with dental decay. It is important to understand how the diagnosis is made. Hopefully, the discussion below will help in understanding:
I cannot see a single spot, but why did my dentist say that I had cavities after the exam?
You may not see any spot in the grinding or occlusal surfaces or the sides of the teeth that are visible clinically. Still there can be decay areas inter-proximally in the contact area(in the area where the teeth touch each other). The dentist can detect these clinically with an explorer or a sharp instrument only if the lesions are big enough. If not, that has to be determined by reading the x-rays. The contact areas usually get decayed first. These areas show up as black spots only on the x-rays.
My dentist told me that I had cavities. I wanted to see the x-rays. When I took a look and had someone qualified read the x-rays for me, there was no evidence of decay. What should you do?

A dentist must combine the radiographic as well as clinical findings to come up with a diagnosis. A carious lesion that may be visible clinically, may not be visible on the x-rays and vice versa. X-rays are only two-dimensional images.
Therefore, they show the lesions, which are only in between. As a result, small lesions that can be in the pits and fissures of the grinding surfaces or on the other sides of the teeth may not show up on the x-rays. If you already have a huge filling in the tooth and the new cavity is on the side of the filling, the images can get superimposed and give a false impression that you don’t have a cavity. On the other hand, the dentist may not see anything clinically, but can later find lesions inter-proximally on the x-rays. Therefore, if you have any confusion and would like to get a second opinion, make sure you get both the radiographic and clinical evaluation done.
Every time that I go to the dentist he finds cavities. Why is that?
There may be several factors involved, for example:
a)You got serious about brushing and flossing only recently
b)There were previously damaged areas that are breaking down only now. So, don’t feel bad. May be you are trying your best. It was just a matter of time that these areas would need to be restored.
c)You brush but don’t floss
d)You brush less than required (e.g., less frequently and for a shorter period of time than required)
e)You brush and floss but keep on snacking the whole day
f)You drink too much soda, juice, and milk without making sure you rinse it out.
g)You did not get adequate amount of fluoride while growing up. Your teeth are weak.

Note:
You should always get a second opinion if you do not feel comfortable with your dentist’s diagnoses and recommendations.