Christianity - or, more precisely, Jesus himself, as LOGOS - provides the context through which I understand the meaning of my life. It expresses the framework by which I assess what is valuable and what is trivial, what counts as good and evil. It's not simply that I download a Christian operating system and then run it on my wetware; there is a discussion and argument and interrogation and self-analysis and slowly, over time, I own more and more of the Christian world-view. It becomes a central part of who I am. There are some exceptions, some grey areas, some bits where I suspect I will one day understand, some bits where I think I will always resist. But Christianity is the framework; it is, in a real sense, how I navigate my life. It tells me (expresses for me) what way is up.
One way of distinguishing between a reasonable and a humourless atheist is that the former understands this and, indeed, can offer an answer to the question 'How do you navigate your life?' There will then be an offering of their own navigation system, whether that be a different faith or something like humanism or Stoicism. Whereas a humourless atheist will normally try and avoid the question, often trying to argue that is makes no sense - in other words, they don't 'get it' (hence, they are humourless).
Whereas I find that the most interesting of conversations come when we find out what it is that people most value, and how they are then able to talk about them, and to establish common ground and areas of difference. It is the sort of thing that can enable those of different faiths (or different varieties of the same faith, or those of no faith) to come together on common projects.
Humourless atheists can't take part in this conversation. I think they miss out on something essentially human as a result. Of course, I also think that such a position is incoherent and unjustifiable - but that's the point of distinguishing one atheism from another.
Is this a strawman? Are there any humourless atheists? Sam Harris would appear to be one.
Other posts on atheism here.