He conquered Europe basically because he had no choice.
He was a ruler born from the French Revolution and of the new world order that it tried to establish. The other European regimes, all of them monarchies and especially Great Britain, made coalition after coalition to try and overthrow him. Of course he was a military genius and they were not, so he kept defeating them, and the best way to stop them from attacking again and again was to conquer them. Then it was a vicious circle. The more Napoleon beat them, the more they wanted revenge, and the more they attacked Napoleon, the more he beat them and took their territories. Napoleon wanted peace, and that's one of the reasons he had himself be crowned Emperor. Mostly because he wanted to start a dynasty, but also because he mistakenly thought it would make him "fit in" more with the rest of European royalty, and they would leave him alone.
An other thing that came into play was the blockade. When he realised he couldn't invade Great Britain through strength of arms he had a master plan to turn all of Europe into a fortress and isolate the U.K. But for that to work, every country in Europe would have to shut down its ports to English ships. And of course since most countries in Europe resented Napoleon they (more or less) secretely did trade with British ships. For instance, the reason he invaded Spain, which was one of his most humiliating defeats, was simply to get to Portugal, which was openly trading with Great Britain. For instance, the reason Napoleon decided to invade Russia (which was the beginning of the end as we all know) is because the Czar had broken his earlier treaty with him and started trading with the British.
Note : I'm not trying to paint Napoleon as a jolly ole fellow who didn't want harm to nobody. There were also political and economical reasons to his conquest, and more than a hint of megalomania. What is true (and often overlooked) is that whatever Napoleon could have said or done the rest of Europe wouldn't (and they didn't) have found rest till he was defeated for good.