Dear …
Thank you for your invitation to go out __(for dinner/sometime with you/on a date with you/etc.)__. I`m sure we would have a wonderful time together, sharing interesting conversation. I am flattered that you are fond of me in the way that you are, __(name)__. I think that you are a __(wonderful/great/super/etc.)__ person and I care for you a lot. At this time, however, I only have friendship feelings for you, which don`t cross over to the romantic side of things.
It has nothing to do with you personally, that`s just the way it goes sometimes. I am one of those who believe there has to be a mutual chemistry, connection or spark before I am willing to start dating someone. My feelings for you may change someday, who knows? But for now, I would just rather keep our relationship at a friendship level. I hope there will be no hard feelings about this. I still enjoy your friendship and look forward to spending more time together.
‘Till we chat again,