Really? I've never seen disbelievers as being happy. Allah (swt) says:

And whoever turns away from My remembrance - indeed, he will have a depressed life, and We will gather him on the Day of Resurrection blind."

How can you say the disbeliever is happy, because you saw them smile and live out a few pleasant moments in life? At the end of the day the best that can be said about most of them is that they are on the edge of a cliff with nothing but darkness to fall into. The atheists among them have this huge question mark on what will happen after their lives. They will cease to exist and death is not something they care to ponder over. And then those that were given scripture, the loops they have to jump through to overlook the gaping holes in their religions. The un-explained phenomenons, the tying of everything to their current situation (which btw your post hints of this too). Surely if good comes to them, it's because they're amazing. If evil befalls them, fear, God must be angry!

We as Muslims know that both good and evil are a test, they can be equally good and bad for you, especially being given wealth and power. If you do not pass that test and abuse that position, you have potential to be in the worst of places come the day of judgment, but a Christian, he will say God has smiled upon him, surely he is doing something right. Little do they know however.

Also I don't know where you are, but here in America I don't think I've seen a non-Muslim who gave me the impression that they were happy, ever. Rather their language is commonly angry and has profanity sprinkled over it like it's normal. Gossip and the discussion of others is very common. The situations they get themselves into thanks to their antics are beyond ridiculous, and while they laugh them off and to you they might seem uncaring, it really often is a humiliating thing they're not proud of deep down. Perhaps you are not seeing deep enough into these people and really taking things are the barest face value, but the matter is very obvious to me. The disbeliever is in darkness at the end of the day:

Or [they are] like darknesses within an unfathomable sea which is covered by waves, upon which are waves, over which are clouds - darknesses, some of them upon others. When one puts out his hand [therein], he can hardly see it. And he to whom Allah has not granted light - for him there is no light.

How can you then ever think that deep down they do not have fears, worries and anxieties? And in today's time, when all of them are in debt and paying off mortgages? His entire livelihood can be shattered in a day if he loses his job, the debts will pile up. They live on the edge in every sense of the word. Just because you see them laughing it off and enjoying some moments? And what about all the different insecurities that result due to their lifestyles? The mixing that can result in cheating? I've heard some non-Muslims in a conversation one time saying that a spouse that never cheats is a luxury, not a given. How many of their rich are suffering, just look at actors, and look at their politicians. You would think with piles of money you would not need to do drugs, frequent prostitutes and fall into scandal. But no, even with mountains of riches it's not enough.

Believe me, Islam is 100% correct in this matter and it is so obvious. A lot of times we read about things we will never see, like the punishments of old nations, or miracles, etc. But this is one of those things that really convince one of the truth of the Qur'aan.

The final nail in the coffin though is that on top of all the complete garbage that comes with the world today. On top of how things go wrong, lives lost, people get sick, on top of all this. It is the heaven of the disbeliever. This is what they settled for. Even if they had it all and really were as happy as you perceive them, this is all nothing in the end anyway. But... they don't even really enjoy it much here either.

But what is the enjoyment of worldly life compared to the Hereafter except very little.