First of all, not only am I correcting your entry but also to teach you and others what to write and what NOT to write in such formal business letters.

A Letter

I am extremely glad to receive your letter informing me that I had been accepted for a position in your company.  This has more sincerity and professional.

Although a perfect job for me, I most respectfully have to decline it due to the many reasons beyond my control at the present.  I sincerely hope that you will keep me in mind in the future.

I thank you for your time and patience.

The job really suits my skills and interests. Unfortunately, after considering both advantages and disadvantages, it was not possible for me to accept your offer. Firstly, living apart from my loved ones would be unbearable and would certainly affect my performance, especially because I don’t have any friends in your country. Secondly, although I’m easygoing and open minded and I know that learning about a new culture is fascinating, I will always feel like a foreigner and I will miss simple things in my country such as food. Finally, my beloved country needs the skills of its competent and qualified people who should contribute to its wealth and development. That is why I do not like to leave it despite the allure of living abroad / of the West, but I hope to visit your country some day to learn more about its culture and to find out how different people live and behave. Thank you again for your encouragement.

You shouldn't delve into such detailed reasons.  I feel that these reasons are extremely weak and that it cuts off your chances in the future.  Why apply for such a position in the first place if you misses family, friends etc.  It shows that you are testing the market and waste the time of the potential employer.  I will never hire such people.  Shows no commitment to anything.

Yours faithfully, sincerely You are not someone who is providing the service.  This is an archaic way, still used by attorneyes and subjects to the throne!

A Letter

The language is too flowery and lush - we would not write such a letter using so many exaggerated words - we use more understated language.  We would also keep it brief - a NO is a NO already.  However I will treat it as an exercise in English rather than an inappropriate letter, hoping no one ever gets to see it in reality!

Nothing really wrong here except the style and the tone