Let me intervene a bit here if you allow me. First of all the "Kuffar" meaning someone who doesn't have a religion or believe in God and it was referred to at in the Qura'n as those who used to worship idols. Also the Qura'n stated clearly like this verse"Those who believe, the Jews, Christians and Sabians - any who believe in God and the Last Day, and work righteousness, shall have their reward with their Lord. They need not fear, nor shall they grieve."
Also, there was another example from the prophet life where he paid respect to jews like this story, that as a funeral of a Jew passed before Prophet Muhammad as a sign of respect he stood up. In doing this, he showed respect and shared in the feeling of sorrow with Jewish family and community. "Why did you stand up for a Jewish funeral?" he was asked. The Prophet replied: "Is it not a human soul?
In addition, personally I don't see a harm of sharing joy with others, in fact this is what (deen= religion) is all about which is "how you treat others" note that others has no specific religion, also I don't see the reason why I shouldn't respect other religions and be with them during their celebrations, just like when they are with me during my celebrations in Eid or Ramadan.
One last thing, think about it, if God wants it to be all Christians,or all Muslims or Jews, he could right? but there is a reason why it isn't so and that verse is a proof " “If thy Lord had so willed, He could have made mankind one people: but they will not cease to dispute” or this one"mankind! We created you from a single (pair) of a male and a female and made you into nations and tribes that ye may know each other (not that ye may despise each other). Verily the most honoured of you in the sight of Allah is (he who is) the most righteous of you.”

That's only my opinion of course, I am not trying to enforce it and this thread is not about religion, we can talk about this more