Most people spend a major part of their adult life at work, job satisfaction is an important element.

It is a well-known fact, that most of the people spend a lot of time working, therefore it is vital for them to feel satisfied from what they are doing. (..get satisfaction from..)
In my opinion to feel personal pleasure from your job is really important. Otherwise, going to the workplace would become just a habit so and people would loose any motivation to do it.
On the one, hand if an individual likes his their profession and does not work in it because they have no choice does not have any other choices, the chances to feel satisfaction are increased.
On the other hand satisfaction is a complex structure. It contains many factors. For instance, it is crucial to be in have a good relationship with your bosses and colleagues. Also, to get a salary that you deserve, and last but not the least - to try to improve yourself through the career ladder. Furthermore, I believe that, if ones  (someone) enjoys his their job consequently, he they would put more effort in everyday. And that would help him them in reaching his their business aims. (goals)
Nevertheless, nowadays most of the people do not look at their work abilities in a realistic way. Therefore they might get dissapointed. However, these one, who know the maximum extent/level of their skills has bigger greater success in their careers.
In conclusion, the role of that satisfaction plays (you must complete the metaphor) in from what you are doing for a living is enormous, because otherwise, you would loose much precious time of your adult life. And someday, if you look back to the past, probably you would probably regret about the missed opportunities that you have had.

I think you need to simpify. The sentences are too long.

Here is an example of one of your sentences written in a more concise way.

In conclusion, the role played by job satisfaction is enormous. A big part of your adult life can be wasted. Someday you might look back and regret the missed opportunities.

Most people spend a major part of their adult life at work. Job satisfaction is an important element.

It is a well-known fact, that most of the people spend a lot of time working, therefore it is vital for them to feel satisfied with what they are doing.

We don't normally put a comma before that. Adding one makes you seem uncertain.

The people is a specific group of people. Because you are speaking in general, just use people.

What they do indicates a general habit. What they are doing suggests this moment now.

In my opinion, to feel personal pleasure from your job is very important. Otherwise, going to the workplace would become just a habit, so people would lose any motivation to do so.

Really is colloquial adverb, and makes written text look a little childish.

Writing "loose" instead of lose is a common mistake amongst native English speakers, and a very embarrassing one. Have a look here for common misspelt words:  http://theoatmeal.com/comics/misspelling. If you get them right, your basic English will be better than most native speakers. ;)

On the one hand, if an individual likes his profession and does not feel pressured to work it, because he does not have any there is no other choices, the chances of feeling satisfaction are increased.
On the other hand, satisfaction is a complex structure. It which contains many factors. For instance, it is crucial to be in maintain a good relationship with your bosses and colleagues. Also, it is important to get a salary that you deserve and last, but not the least, to try to improve yourself throughout your career ladder. Furthermore, I believe that, if someones enjoys their job, they would consequently put in more effort each day. And That would help them in reaching their business goals.

Okay, there are situations where we do add a comma, and these are called prepositional phrases. Without going into it in too much detail, basically if you can cut a short phrase from the beginning of the sentence without affecting the main statement/meaning of the whole sentence, then add a comma. :)

Note I used they/them/their to indicate a gender-neutral single person. This is a fairly new convention.

Never begin sentence with and or but. It is lazy writing and marked as wrong in an exam.

Nevertheless, nowadays most of the people do not look at their work abilities in a realistic way. Therefore As a result, they might become disappointed. However, these ones, who know the boundaries of their skills have greater success in their careers.

Just a style point here: I think there are too many cases of using nevertheless, therefore, however, and other such phrases. The effect is that the reader feels pushed around. I suggest to use them sparingly, and instead aim to write a clear and solid paragraph without all these "sharp corners". 

In conclusion, the role of satisfaction in what you are doing for a living is enormous, because otherwise, you would lose much precious time from your adult existence. And Someday, if you look back to the past, you would probably regret about the missed opportunities which you have had. 

Regret is a transitive verb, meaning not only must you tell us what is regretted, but no prepositions are necessary. ;)

Hope that helps... I am being very thorough, so don't feel too bad about the corrections and comments. Keep at it. :)