MrsRisotto - it's an interesting question.
Firstly, I guess there's the specifics - so, is the idea of fasting or a time of purification historically a religious concept (note that I'm saying 'religious' rather than Christian)? I think that this is probably the case, although it would be interesting to see an argument against it by a historian.
Secondly, the broader point. So, are some people who actively practise a religion resentful of people who don't practise that religion adopting some of the cultural practices in a non-spiritual way? Well, I guess that there's such a huge range of religious people, faiths and opinions in the world that some are probably resentful, while others are absolutely delighted. So from that aspect, you could probably find people to agree with you and others who would disagree.
I would guess that some Christians think the idea of giving up chocolate etc for Lent without having a spiritual aspect to it is a little bit odd - what is the difference between that and an ordinary diet? And if you dont' believe in Lent, then it seems odd to do something to mark it. But, on the other hand, I do appreciate my colleagues joining me, and I'm told that lots of Muslims celebrating Ramadam really appreciate their colleagues supporting them in that.
Does that help give a broader context on your thoughts?