You're not the only one who picks decks that may not be tier 1 for psychological reasons, and I don't think it makes you any less of a competitive player for doing so. You're not an AI playing this game, you're a human, and part of the challenges you'll face come from within as well as without. I don't compete as heavily as I used to, but I've definitely picked lower-tier decks at competition levels as high as Worlds.
Personally, there are situations in which my mood will tank, which has a tendency to be a feedback loop and can ruin both my enjoyment and performance. Losing can lead to lower mood, can lead to losing, can lead to... etc. (I know there's a term for this in competition, I'm blanking on the phrase right now.) (EDIT: "tilting" is the word I was looking for!) Playing a deck I know better, enjoy playing more, or take some special degree of pride from can help moderate the lows, elevate the highs, and prevent my mood from tanking. I'm a better player when I'm in a better mood.
I still like competing and might be happiest coming in first with any deck, but coming in second with a pet deck is pretty dang close. In contrast, losing with the "best" deck feels worse than losing with a pet deck. These things matter different amounts to different people on different days, but they're worth considering.