Yes. I completely know how you feel. I constantly have to fight the completist compulsion. I've had it strike on toy-lines that I don't really even enjoy. For example, I've never understood the Hot Wheels collector; too much stuff to collect (yes I know, from a Star Wars collector, that's black kettle, black pot territory). But, every now & then I'd pick one up that had some sort of personal reason behind it, i.e., my late grandfather used to drive one of these pick-ups and now Mattel made a small die-cast version ... so I buy it. Then I notice that it just happens to be car #3 in a 12 car series, so I go out and buy the other 11 in the series. And why!? I don't know.
I'm able to resist it much better these days, probably comes with the maturing process, but every now and then I still feel it. I don't know this for certain, but the compulsion is probably very similar to OCD or the psychological addiction that a lot of people feel (not the physical, mind you) about things they repetitively do without knowing why they do it. This does not apply to those who decided to be completists, think Sansweet, but to those who are completists because they can't stop themselves, they're compelled to buy the next figure, vehicle, playset, Mpire, plushy, etc..., even though if they really examined themselves they'd realize they don't really need, or want it.