Why so many thought in one's mind?

Sounds like racing thoughts.

Yes, it sounds like racing thoughts. If it bothers you to the point that you can’t sleep at night, a doctor might prescribe a mood stabilizer or anti-depressant to lessen the effects.

Anyone who has done much meditation knows all too well that the brain is constantly spewing out random thought activity. That’s just what it does. How troublesome that is depends mostly on how you use attention.

That thought chatter is just one possible “channel” your attention can tune into. Sensory inputs are another (including your sense of your own body). Deliberate, disciplined thought is another. If your attention locks into the mind chatter, you feel swept along in this crazy tumble of images, fantasies and feelings. But you can exercise the executive control functions of your brain to focus your attention away from that chatter and onto one of the other channels. Meditators often do this by focusing in on the breath, but you can do much the same by opening up to your surroundings, or just getting in touch with your body.

Note that tuning into these other channels doesn’t mean that the chatter stops; it doesn’t. But it no longer becomes compelling or relevent, and isn’t perceived as a problem.

The mind’s nature is to produce a river of thoughts. This is only a problem if you identify yourself with the mind. If you look, you will see that you are an observer of the mind and its thoughts and not the mind itself. This space allows you to turn your attention towards or away from the mind and its thoughts. With some effort and practice, you can unseat the primacy of the mind so that it better works for you rather than you for it.

The brain is very active. It can actually hold 11 thoughts at once, some contradictory.

The brain is most active & sensational part of our body. There are lots of collection of thoughts, which are moving in our surrounding. Our eyes & brain is connected with each other so whatever eyes watches, it will make an image in our brain. Hence, our brain think lots of things in a same time.

Dig em out starve them out crowd them out Like weeds in a garden you must decide which ones you want to keep and focus on them

The root causes are not fully understood.

In general, we can view the brain as an association machine. Neurons that fired together in the past, will fire together in the future.

Actually, the mind exists because of goals. It always chases after the goals. If you could somehow remove all the goals, the mind ceases to exist. Sounds weird, huh?

The philosopher Sartre had an interesting comment on this. He said that the conscious mind is always conscious of something.. There is no such thing as pure consciousness. In an interesting twist on Descartes, Sartre said that we become aware of ourselves as what is left over from the content of our consciousness, what Sartre called the hole in Being.

Like @thorninmud suggests, if you want to become aware of all the chatter going on in your mind, try meditating.

On average, we have some 70,000 thoughts every day (I read that somewhere). I envision a mind as having a collection of ‘egoettes’ with each having the function of seeking fulfillment of individual desires. Usually, because they operate subconsciously, one is not aware of their existence. However, the minds of some people give their egoettes a voice. Buddhism teaches that desire is the root of all suffering and that by eliminating desire all suffering will cease. As @thorninmud and @LostInParadise suggested, his can be attained through meditation.