I can see why many people in Europe would be alarmed about the situation with hundreds of thousands of refugees trying to get into Europe from Syria and other troubled countries; especially after the terrorist attacks in Paris. I would guess that, by far the vast majority of refugees are totally decent people; like you or me. Totally non violent. It just takes a handful of people, for instance 1 out of 10,000, to cause problems and spoil things for everyone. It's a scary situation even though practically all of the refugees are good people. Scary situations can lead to backlash as some folks react out of fear wanting to close borders and so forth.
I've heard that the British are trying to pursue a policy of helping Syrian refugees in places closer to their countries of origin; such as helping Syrian refugees who have escaped to Jordan. In the short term, at least, this might work better than taking in large numbers of immigrants right now. Taking in immigrants over the long run can be good when there is time to do it wisely. Also societies will need to function so new populations aren't isolated into ghettos. Find ways to integrate into the diverse societies.
In the long run, taking in immigrants does bring more young people to a society; for instance here in USA our immigrant population is providing young workers to pay into Social Security as our post war baby boom bubble reaches retirement age. Europe's aging population could benefit from this also; especially if it happens gradually. The current situation is more of a shock to societies, however.
Even here in USA, where we have the benefit of being across large oceans from most of the world's trouble spots, people tend to be nervous about large numbers of newcomers. It's kind of a human trait. There are people, here in Bellingham for instance, who don't want Bellingham to take on new residents. Not just immigrants, anyone. There are people who don't want our city to grow. As long as world population keeps growing, popular towns and cities, such as Bellingham, are bound to grow.
Setting up safe havens for people trying to escape the trouble spots seems like a good idea. What role the military should play is being hotly debated, of course. Not just our military, NATO and so forth. Most of the military involvement seems to make things worse, but there could be some better strategy. Possibly the military could be used to take some territory for setting up safe havens inside former Syrian land. Maybe help the Kurds set up areas that are more tolerant of the diversity in that region. This might help take some pressure off Jordan as Jordan isn't very large. The Saudis haven't been that helpful for sure and they could be part of the problem.
I'm just throwing out some ideas, but realizing any strategy could be problematic. I can see why people are concerned. It's hard to figure out a proper strategy.