But i guarantee you on this you will be happy when you die. ...
No, you actually don't
. A guarantee carries compensation in case the thing that's guaranteed fails. For example, say your lawn mower is guaranteed not to break down for the period of a year. Typical compensation would include, say, the repair of that lawn mower, in cases where it does indeed break down in under a year.
You aren't offering any compensation, so your guarantee is totally empty and void of responsibility on your part. You aren't able to say to curiousgirl, "Oh, if you die and I was wrong, I'll give you a cookie!" - because curiousgirl would be, you know, dead
What you are offering is a promise
, not a guarantee. And a promise from you on this topic carries no weight at all
, because you havn't shown that you know that anything you're saying is true. Especially since you can't even speak from personal experience.