Why do people including me have low self esteem? Is it because of some terrible experiences?

Not always! If there are terrible experiences then it will definitely contribute, but it's not always the cause. Often low self-esteem stems from what's called 'core beliefs'. These are basically beliefs that we have about ourself. These were often just a floating thought but now in our mind, they are facts. This might start out as an innocent comment from a relative, a tease from a friend or even a magazine article that suggests you're not good enough.

I have low self esteem, beat myself up needlessly, from time to time, but I don't believe it's due to one terrible experience, but rather a lifetime of itty bitty occurrences and my own self-doubt clouding how I percieved others percieving me. I tended to always think when someone looked at me, or was laughing near me, they were laughing at me or mocking me, but in reality, people are typically very egocentric and unobservant, and when they were staring in my direction, they were probably daydreaming about cats in sweaters made from their cat hair, or laughing at the image of a cat wearing a sweater made from their cat hair.

It can be due to many different reasons, but your upbringing is quite a big part of it. If your experiences and opinions are invalidated, you grow to doubt your own judgement and then doubt yourself, contributing to poor self-esteem.

Low self esteem can be the result of many different reasons. The home environment, people that one meets, failures that exceed successes and a personal view of life.