Now, I'm all for making it easy to immigrate, or at least easier than how the US currently has it (or at least less costly/miserable), but open doors? You're just asking for conflict that way.
Red = Countries with serious conflicts going on, pretty much all of them except for Ukraine related to islamism
Blue = Countries who have skimpy borders and that doesn't mean they're asking for it, we don't country slut blame here
Imagine a party.
The guy on the door won't just let people not on the guest list come in.
A guy wants to go to the party but isn't on the list, so he puts himself in danger. Maybe it's raining so he runs across the road to the house, slips, and whacks his head on the pavement. The door guy isn't a bastard, so helps him into the lobby rather than leave him facedown in the rain. The guy gets himself back up and tries to slip into the party before someone else spots him so he can have a good time, even though nobody wants him there or really wants him in.
Another guy is taking a smoke break outside the next party over and is attacked by some thugs. He runs for the safety protected by the huge guy at the party's door and the door guy lets him in so he isn't beaten up by the thugs (They later throw firecrackers over the wall into the party garden). The guy realises that this party is much better than the one he came from, and as long as he can't go back to the party all his mates are at because of the thugs, he should check out the real party instead of waiting in the lobby with all the guys who ran across the road and knocked themself out trying to get in.
The EU is a big neighbourhood. Lots of families have their own homes cobbled together with their own front doors and walled off gardens and such, but one day the Europeans get a bright idea - what if they knocked down all of their doors and turned all of their houses into one super-apartment. Sounds great, and if anyone disagrees you just ignore them and keep knocking down doors anyways. Things get better as all the students realize it's really fun going through everyone elses houses without having to ask for permission, everyone trusts one another anyways so it's no big deal.
The surrounding neighbourhoods were absolutely fucked, and got even more fucked as time went on. The sheer levels of fuckery that occurred could not be detailed in one simple post, no the fuckery could not be contained, spreading into other neighbourhoods and causing even more fuckery. Some of the families in the European neighbourhood were even involved in the fuckery, returning home to their big neighbourhood with loot and shit.
That's when the European homeowners council start getting complaints. They forgot to install a gate for their little community. Suddenly loads of people start showing up; it's ok though, France can vouch for them. But the volume is increasing further and that's when things take a complete fucking shit turn, tens of thousands of families moving in from Africa, prompting the German family to say everyone who arrives from Syria is welcome, especially Syrians from Africa who have never been in Syria before
The European homeowners spend so long arguing with the European homeowners council that nothing gets done and the Serbian family is panicking telling everyone else that jihadis are moving into the apartments. The Austrian and the Hungarian families are busy boarding their houses up whilst the British family is deeply concerned with the recent string of murders taking place amongst the European families, in order to placate their fears the European homeowners council decides to hire one Turkish bouncer to police the European community but he doesn't even do his job and just stands by his gate smoking euro notes whilst everyone walks past.
Fast forward 3 years. Most families have determined that the European council association of homeowners is feckless and have just reinstalled their doors. Nevertheless they have to now share their home with four men called Umar and one of them goes on about how he doesn't support ISIS, but understands it, sharpening his machete whilst reading lewd theology. The Swedish family laments that although they fucked their house over and they can no longer pay the bills or stop the house from falling apart, at least they were tolerant. The German family is haunted by the fact that every time a jihad lorry runs into the house, their Grandpa starts quoting mein kampf on his twitter; in response the German family have decided to keep the door open but demand that no one wear burkas inside the house. Them's the rules. The French family have decided too that anyone who wants to use their beaches have to wear bikinis or go nude, that is simply the French way. The British have started a bonfire using sterling notes whilst a grey Englishman simply yells 'we burn sterling you burn Churches doo da, doo da.' The Hungarian family decides that one fence is not enough, they need 2Fence. The Ukrainian family pretends like people have actually noticed them. The polish family shrugs its shoulders in confusion, no migrants ever tried moving into their house, and the door is still open. The Croatians and Serbians remind arrivals that their houses are still full of landmines and they don't know where they put them all.