A sensitivity to little irritating things is a condition. I never really believed that, but I have come to believe it. You can’t tell someone “don’t sweat the small stuff”. That is like telling someone to love.
You can tell someone to not make issues of small things for other people, perhaps because it will make things worse for themselves. But you can’t tell them not to sweat it – sweat it they will, if they are so inclined.
Not being sensitive to little irritating things is a state of mind. It can be prevented, it can be induced, it can be nurtured, but it can’t be decided upon as a policy.
The only thing I have found that helps with this – medication aside – is good health and quiet meditation. It is not so easy to simply pick up meditation only when you need it. It is important that meditation be part of your constant preparation for little irritating things.