If there is no life after death, if existence ends when the brain ceases to function, I shall never have the disappointment of knowing it.
That means that life after death can only be proven in the positive.
If believing in life after death makes this life more meaningful to me, what do I lose by believing?
I will grant that some people may find life less meaningful, for some reason, if it continues. I don't want to take their hope away, but I still think there might be a better thing to hope for.