Regret is to feel bad at what you don't have. Regret is the sadness felt at an unmet desire. Regret is negative. Because it's a feeling of being deprived of what you want.
Regret is like a cloud. A cloud prevents you from seeing the sky clearly. Regret prevents you from seeing the blessings in your life. You only see the cloud. You miss the sky, you miss all the silver linings.
Regret is a poison. It eats your positivity and virtue at the same time. When your thoughts have regrets, its like you have decided to sit in 1 corner of the house and sulk. Everything is happening normally in the house, but you are sulking. You are the only person affected. When you have regrets, and are constantly complaining, the breath of your life loses freshness, and fragrance. You emit a foul odour, sometimes so foul, that other people start avoiding you. The more the regrets, the less the friends. Positivity attracts friends, regretfulness lives alone, it cries at its own solitude.
Sit back, start imagining. Put yourself in the shoes of the 10 most sad, and deprived people you know. Then, think again. Do you now not feel grateful ? Don't you feel blessed ? We miss the 1000's of blessings in life when our minds are clouded with anger, jealousy, and pride. We want more and more. This more has no end point, it is the corrupted lust of greed. This pollution in our minds gets cleansed with the pure fountain of contentment. When this happens, there is nothing to regret.