It is a very complex question, and I will try to be as concise as I can.
I think we need to split the arrival of migrants to the European Union via different kinds of routes, and I think the main reasons for people migrating to Europe would have different backgrounds.
First of all, if we look at the eastern Mediterranean route, which is the link between Turkey and Greece, either the land border or the sea border, we see that most probably the highest number of people come from Syria. That was the case in 2015 and in 2016. Nowadays there is a certain shift in nationalities coming to Europe via that route, but the main reason was the ongoing war in Syria, which sparked a high increase in the numbers of people.
If you move to the central Mediterranean route, the story is slightly different, because the nationality you would see on those boats would come from Bangladesh; you would see Eritreans and Libyans, and most recently, an increasing number of Tunisians and many from western Africa; Nigeria is dominant.
For them, the main reason would be economics, trying to reach Europe for economic benefit. I'm not playing down the conditions in some of those countries for those people, but it is not migration because of war or other disasters.
The most recent phenomenon is migration in the western Mediterranean linking Morocco and Spain. Again, the migrants you would see on those ships would come mostly from west Africa. They are of a slightly different composition than in the central Mediterranean. They would come from Côte d'Ivoire, Senegal and Nigeria. There again it would mainly be economic migration.