To me in order to find the reason we just have to look at ourselves.
Let's say an alien lands in the Netherlands or whatever, says "Hallo," then files for citizenship. You can't honestly expect the entire world to just accept this and move on. In fact, there will always be at least ONE group that will consider the existence of aliens to be counter to their core beliefs, and will attempt to eliminate them.
Following this, we have three types of aliens:
1: the ones that know this, but still introduce themselves despite the threat. That means that they are confident that it will not be a problem, most likely because they plan on taking over our planet.
2. the ones that know this, and so avoid us. This is the best solution, because we will never know their existence.
3. the ones that know this, but have no choice but to arrive. chances are that this is just a guise of group 1, but otherwise they might be able to co-exsist with another species, but more likely one of us will end up destroying the other.
4. the ones that do not know this fact. Either due to politics or lack of analyzation they do not know what will happen when the arrive. this means that when they arrive with peace in their minds and are met with criticism, they could be shocked and believe that all of us are like that. then they may try to destroy us, worst case scenario.