Imagine you are holding a piece of string, it is lovely and light. Then someone begins to tie things to it, some of the things are light and don't really make a difference but others are heavy. The weight on the string gets heavier and heavier, the string begins to stretch and then fray until eventually, it snaps.
Luckily, if you add more threads to the string it will get stronger, every thread weaved into the string makes it able to hold that bit more weight, until eventually you have a nice strong rope. The rope can still snap, but it takes a lot more to make it do so.
Humans are a bit like that string, we can be relaxed and level headed and then something can happen that begins to change how we think and feel, the more that is added, the more likely it is that we will snap.
Some things might might not bother us at all, but others might really push our buttons. Depending on what life has been like for you so far, your current string might have very few threads, or it already might be very robust. For those of us with few threads, it can be much harder to manage day to day than those that have good strong rope.
We too can weave new "threads" into our brain that mean things don't have the same impact on us as before. Overtime you can turn your string into rope, but just like weaving a rope, it takes time and patience with yourself.