How do i behave with a friend who wants to be more than friends but i dont?

Well i have been really good friends with this guy for 5yrs he is a really good person.i have fun with him we go out and he has been there for me through some bad times theres nothing he would not do for me.he treats my daughter so wonderfully and he means alot to me.however when we met i let him know it would never be more than friends and i always felt like he wanted more.everytime that topic comes up i let him know that we can never be more and i dont want to mess up our friendship he always says he understands but he brings it up again eventually.now he sends me a letter telling me the samething yet again.if i could make myself fall for him i would but i cant im not in love with him.Any advice on how to handle this situation?should i end this friendship now?i really dont want to i like him as a friend especiaaly when i am lacking in the friends department.