Relationship between friends is very important and things that make this relationship stronger are plenty. It is common for friends to send gifts to each other in the hope of building trust, but when one of your friends has done you a very special favour, you have to write a strong thank you letter, letting him/her know that you understand his/her importance.
Start the letter by saying thanks to your friend for whatever he/she did to you. You can briefly mention what really made you happy and the reason you are writing this letter. Do not mention every single detail in the opening paragraph.
The second paragraph can go into a bit of detail of what happened and how much it made you happy. You can mention briefly the details of the matter/issue. If it was a favour, explain how things got underway. You can also mention the importance of your friend’s support to you.
Conclude the letter by expressing your gratitude for your friend once again.
Do not forget to add closing salutations like “Your Sincerely” or “Kind Regards.”