I keep counting to 7?!!!?!

I keep counting to 7?!!!?

Ever since I was about 10 yrs old I have this compulsion to count to 7 in my head! I could be driving along in my car and I start counting the cars on the opposite side of the road and they have to add up to 7, I do it with people and I have to be the 7th person, I do it with clothes and add different sections so that they reach 7 but each piece has to be attached to each other in order to continue, I do it with faces and each section has to be joined in someway in order for me to count it in as part of the 7.
I do this the whole time and have actually become an expert at getting and rearranging things to reach 7. Nobody know's I do this as I do it in my head and mostly when I am on my own or if I am bored within a crowd or stuck in bad traffic etc, sometimes I'm even doing it unknown to myself.
Why do I do this? Or can anybody explain this to me?

God, I though it was just me....... people say it's OCD but I remember doing it as a very young child when life was great, no worries, great family and no one I loved had ever died - it must be a genetic thing I guess caus emy mum and I have discussed this and we both do it.... it's part of who you are, just go with the flow.