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All are advised to read"Are you really healthy" by Radhika Kinger. It's an eye opener. You become what you eat . you should eat as your stomach requires & not as your tongue asks. As Sri Sri Ravishankar says "Discipline brings freedom in long run" e.g. brushing your teeth is a discipline but it brings you freedom from tooth problems in long run so one has to have discipline in eating to have good health in long run.
As per the above mentioned book, 90 % of food we eat today is wrong, SO GO BUY THE BOOK.
To give a direct answer to your question, jeene ke liye khana jaroori hai bcoz food is one of the sources of energy that body can't do without and whatever you do should be source of joy both in short term & long term. more importantly in long term.
Well noone would have expected such a profound answer to such a simple question.