We all have some degree of sincerity and some degree of showing-off. But as Matthew says, there is no reason to assume that someone is just showing off.
I do sometimes feel that unconscious irritation, while knowing it is my problem and not anyone else's. A lot of people quote random things from Catholic books on Facebook. Since if I did this, it would come off as affected, it makes me thinks others are a bit affected when they do it; but that is not the case. God gives certain people different things to say, and a different time to say it. Certain saints kept silent for long periods and then began preaching all over the place later.
I personally don't understand why someone would choose one paragraph instead of another to post on Facebook, almost randomly, where it is deprived of any context. To me I get very little spiritual nourishment out of it, because I'm more of a big-picture thinker, I have a conceptual mind. But can I say that they are fishing for a "like"? No, they have their reason for what they're doing and it may be a good one. Maybe God gave them an inspiration to choose a certain quote, knowing someone will see it who needs to see it on that day.
So many times there have been things that rubbed me the wrong way, even behaviors of the saints. I had to force myself to see it closer to God's way, instead of mine, to appreciate the differences between His children. Human arrogance is such that we tend to want everyone to be like us, like seeks like, but God didn't plan it that way. Two saints I had problems with are St. Therese and St. Francis. With St. Therese, for a while I was uninspired by her "little flower" image, thinking it was sentimental, while our time demands soldiers and scholars, etc. But I was totally wrong, now I see why she is a saint. So it always pays to be patient and not leap to judgment.
St. Francis is another example. St. Francis really speaks almost like a baby when it comes to God and His creation. "O brother sun, sister moon" and things like that. My natural instinct hearing things like this is to cringe. Imagine you are in a room alone with someone, and then he looks up to the sky with milky eyes, saying "O sister moon, how I love thee!" You would start squirming and be like "Okay, dude, chill out."
We are so denatured that we can't understand someone who has reached this level of total innocence. That is because we are all affected with human respect. St. Francis had conquered human respect to a degree that is almost impossible to believe. He is basically the paragon of humility, opening himself up for mockery constantly.
I spent the first thirty-two years of my life trying to be as cool as possible, and St. Francis is severely uncool -- the epitome of uncool. His sincerity is so incredibly profound that to children of today's ironic age, it's hard for us to believe he really was like that. But that is the way we are supposed to be, innocent, full of wonder of God, like children. My "coolness" is of the devil, it is a mocking, worldly spirit. St. Francis is simply spiritually light-years beyond me, which I have to recognize. I can only pray that one day I will have a fraction of that sincerity and simplicity.
So in many cases, as Matthew points out, our irritation is our problem; it is our worldliness, our human respect, our desire to make everyone like us, to not see when they are superior to us, because we think we're it, that we know the score better than they do. We don't want to see that they may be far more advanced spiritually than we are.
There is a reason Christ constantly emphasizes humility, if you are self-satisfied, you will never unlock the riches of the faith, because you can't learn, you will just be stuck in your rut. Take the last seat at the table, assume others are better than you, and you will quickly have entire new vistas opened to you, will see things in a whole new way. If someone does something that seems odd, there may be a reason, or a lesson.