Well, I did too. Now that I am a little older I understand where some of their objections stem from. They don't celebrate Christmas or Easter because they are not in the Bible and have more to do with paganism than Yeshua.
They don't celebrate birthdays because that concept isn't really biblical either. The only birthday celebration recorded in the Bible is the party for Herod Antipas and we know where that went. (Matthew 14:6-12)
I never read anything about dourness or lack of celebration in the Bible... so that never did make any sense to me.
Christians need to understand that a lot of the things they do aren't really biblical. Birthdays parties, Christmas, Easter have more to do with paganism than the true religion of the Bible.
I think the only reason that the JW religion is so "dour" as you put it is that they don't celebrate G-d's true holy days as spelt out in the Torah. If Christians and JW would celebrate those, they would have more fun, not less fun.
> They don't celebrate birthdays because that concept isn't really biblical either.
Calculus, computers, airplanes, religious freedom and democracy aren't exactly Biblical concepts either.