What should be the attitude, the mindset, when playing boardgames? Do you play mercilessly, trying to win at all costs? Do you play nice and just try to make sure everyone has fun? Do you go as far as cheating to try to win? Do you treat different players differently, being nicer to some (your spouse, your girlfriend, or someone you are trying to charm) but tougher on others (other players perceived as being stronger, or your spouse?!)?
When I play, I generally do my best to win. I do exploit mistakes or weaknesses of others. I work hard to win. I don't go soft on my opponents. I expect the same of my opponents. We should all do our best. In fact, This is a matter of respect to your opponents. It is when you do your best that you get the most enjoyment and excitement out of the game. It is the battle of wits that I enjoy in boardgames. Immersion into the theme of the game is nice, but it is the competition in a risk-free world that is fun.
That's the general rule for me. Here are the exceptions.
Sometimes I do play nice. E.g. when playing some games with my wife, like Ticket To Ride. We have an unwritten agreement on the playing style for this game. We prefer not to intentionally block each other, and only focus on connecting the cities we need to connect. Blocking others in this game is perfectly legal and is also a valid strategy, just that it is not "nice", especially when you are the victim. By playing nice, we do lose a bit of the tension of the game, but we are OK with that. But when we play Carcassonnne, we play pretty mercilessly.
Another situation where I play nice is when teaching new games to people. I don't intentionally try to lose. I don't intentionally make bad moves. But sometimes I don't think too much and don't try too hard to come up with the best move. I play a little more by gut feel and a little less by careful planning. I will also point out any obvious mistakes made by the new player, I allow him/her to take back his/her move, I give some strategy tips, but not too much. A new player should explore the game and enjoy the process of exploring the game.
One thing that I do when there is a rule dispute, or some rule ambiguity, or some detail rule that I cannot remember or cannot find quickly in the rulebook, is that I will decide on the interpretation that will disadvantage myself, or will advantage my opponent. I just want to get on with the game and I do not want to be stuck in a rule argument. I can always check back the rule later. No point allowing a minor rule to spoil the fun. But of course if it is a rule with huge implications then I will try to find it in the rulebook to get the right answer.
I seem to talk more about the exceptions than the norm. Seriously, I play to win. But as Reiner Knizia says (gosh, I'm sounding like a worshipper), “When playing a game the goal is to win, but it is the goal that is important, not the winning”.