Cheating has a rich set of reasons, and any one cheater may be doing it for a mix of them. For some it is to show power over other (like to make them unhappy in your example), for some it might be a sense of entitlement (I've paid for this game I should have the right to win it how I like), in some games there are people paying others (in game tokens or real world money) to level up for them and those doing the levelling up are using cheats to improve their profitability, and a number of other reasons.