So as quite some people know on here, I am strongly against closed source software and have a very strong distrust in it as well.
So next to some principles (and believes etc etc etc) there is one specifc 'event' which triggered the distrust in CSS (No not Cascading Style sheet, I mean Closed Source Software :P). So hereby the story about what happened.

I think it was about 5 years ago when a guy joined my programming class (I wasn't in uni although I studied but for the sake of clarity, lets just call it uni for now (also, that makes me feel smarter so why the fuck not!)) in uni. He knew a shitload about programming for his age but he was convinced that he was always right. (that aside)
Anyways, at some point we had to work in groups on this project (groups for specific tasks) and he chose (he loved it, we hated it, he had the final say) Trello for 'project management'. He gave everyone (I was running Windows for a little bit at that moment because the project was in C# and the Snowden leaks had not arrived yet so I was not extremely uncomfortable with using Windows, just a lot) this addon program thingy he created for Trello which would make usage easier. I asked if it was open source, he replied with 'No, because this is my project.' and although I did understand that entirely, I didn't feel comfy using it because of it's closed source nature. Everyone declared me paranoid and he was annoyed as hell but I just kept refusing to use it and just used the web interface.

*skips to 2 years later*

I met that guy again at the train station at a random day! Had the usual 'how are you and what's up after a few years' talk with him and then he told me something that changed my view on closed source software for most probably the rest of my life.

"Hey by the way, do you remember that project of a few years back where you didn't want to use my software because of your 'closed-sourceness paranoia'? I just wanted to say that I actually had some kind of backdooring feature build in which (I am not going to say what) allowed me to (although I didn't use it) look at/do certain things with the 'infected' computers. I really wanted to say that I find it funny how you, the only one who didn't give in to my/the peer pressure, were the only one who wasn't affected by my 'backdoor' at that moment! Also your standards towards the use of closed source software probably played a big part probably. I find that pretty cool actually!"

Although I cannot confirm what he said, he was exactly the type of guy who would do this IMO (and not only IMO I think).

So yeah, that's one of the reasons AND the story behind a big part of why I don't trust closed source software :).