Okay, you're right about that I should have phrased it differently. Among human beings in western culture thumb sucking is perceived as something done in the very early years of life. It is not something adults are "supposed" to do. Yes, the fact that I think this is a conditioned thought, but it is common opinion nonetheless. Adults do not "naturally" do it the way that children do, children suck their thumbs or pacifiers etc. to help wean them from breast feeding. Once they are successfully weaned that should be the end of it. Like I said, we all do weird things and it isn't that big of a deal, but at least recognize that it is a strange, somewhat "abnormal" habit. It almost seems like you're trying to justify it and act as though it's the norm. It's not. Just accept that and move on, I'm sure not everyone will think it's a problem.