Why do I sometimes put objects (toiletries in the bathroom, ornaments, garden plants, etc.) together in two's?

I have done this on and off since a child. I had a dysfunctional upbringing (domestic violence amongst my parents) from which I have blossomed from an angry person into a pillar of the community and a nice person, after various amounts of counselling of sorts. I can't believe I am still doing this though as I am a very strong person. I sort of know myself but was even too embarred to explain to a counsellor about it. I don't always do it, sometimes I don't even think about it.... it just creeps back. What do you think? I'm sure some of what you say I already know... but it all helps to know what others think. Thank you.