(I know, this is a mostly asexual blog, but I'm also a nerd and a gamer, so put up with it)
Back seat gaming has got to be one of the most annoying things someone can do, yes? That a physical person in the room with you who is (usually) just watching you play the game can do, at least. For anyone who's got not a clue about what I'm talking about, firstly you are either not a gamer, never play around other people ever, or so goddamn lucky, and secondly, it's when someone who is not playing the game with you tells/tries to tell you what to do in a game, without you asking or indicating you want their help. E.g. - "Please tell me how to shoot a portal on that wall" "Well you have to go over there and portal up to there and then..." is not back seat gaming. You have asked for help and they are giving it to you. Back seat gaming is when you're playing a game and someone watching starts saying "No No No you're doing it wrong you have to go over there there's no point going there to solve the puzzle you just flip the middle leaver twice and the right one once..." and so forth. You did not ask for help, and they have stopped you working things out for yourself and/or discouraged you from exploring.
Most of the time, it's not an intentionally malicious thing - the back seat gamer in question probably thinks they're helping you. However, I tend to find it's like someone who tries to 'help me' read a book by telling me the entire plot when I'm three pages in. It totally spoils the book - now I know what's going to happen, so nothing's surprising. A game example is like if you were playing Portal, and someone walked by and told you how to solve the puzzle just as you walked into the test chamber. More than slightly annoying.
Except with games, I find myself even more annoyed by back seat gamers, because they tend to make me feel like I've cheated -I get no sense of accomplishment from doing exactly what someone else told me to do. It just followed instructions, I didn't work it out for myself or have the chance to try before deciding I needed help.
And, I find, I get really annoyed and kinda hurt because it feels like they're telling me I'm not good enough, not smart enough, to play a goddamn game. I feel like they're saying "Wow, you suck, here, let me help you because you'd never be able to work that out on your own, and there's no point in letting you try because you won't be able to do it". It makes me feel like they think I'm not capable.It makes me feel doubted. And I get enough of that in real life - I really don't have any reason to have to put up with it while I'm just trying to enjoy myself in a hobby.