Apart from the money involved that could make gamblers feel remorse or regret, I think for a gambler why you should feel that way is when you didn't follow your instinct and you bet contrary to what you want to bet maybe a friend caused you to change your mind in the middle of betting what is from your mind and at the end, it happened that you would have won the bet if you had followed your instinct and analysis.

You can also feel remorse when you have almost won the bet bet in few minutes there is a turn of event against your bet like if a good was scored against you in dying minutes of the game.

You can also feel that way if you eventually won the bet but you staked with very low amount, so you would be wishing you added more funds to your bet that would have increased your winning prize. On the other hand too, vice versa, you can feel that way if you took higher risk and lost the game.

I think majority of bettors regret when they have cash out to pick up but because have greed, they leave it in expectation of a bigger potential win but to no avail, they lose everything at the end when the cash out is gone and the bet is lost also. Many bettors have this remorse certainly.