Most people will lose at poker long term because it is a couterintuitive game, in almost every respect.
Bad player thinks: play as many hands as possible, as cheap as possible. Seems logical, right? Wrong!
Bad player thinks: I should limp my AA, KK, so a can get alot of money from these guys, right? Wrong!
Bad player thinks: I don't need to control my ego and competitiveness, it's good for my game, or at least won't HURT my game, right? Wrong!
Bad player thinks: I just lost $20 playing 5nl, so I'm gonna jump up to 10nl/25nl to get it back, right? Wrong!
Bad player thinks: I am just going to minraise the limper preflop so I don't have to risk as many chips if I don't hit my hand, right. Wrong!
Bad player thinks: I'm going to slowplay my set / 2pr on the flop. Good idea right? Wrong!
And so on and so and so on.
Until a player learns to reprogram his "instinct"/"intuition" he will lose money.
Most people start out in the very beginnning with this basic (losing) mindset.
If most bad/new players just did the complete opposite of what their "instinct" was telling them to do, they would prob do a ton better. Without even knowing why.