Every Rose Has its Thorn

There is a saying that goes something like, "well if that isn't a thorn in my side. . ."
And from the context in which it (and similar phrases) are used, you can tell it's talking about something that probably sucks. You know, because thorns hurt. And if one is in your side, that would probably hurt really badly.

I guess what I was thinking about earlier this weekend was why a random thorn would be in someone's side. Wouldn't it be more powerful or descriptive to talk about like, say, a shovel in one's side? Or a knife? Pitchfork? Or are we assuming that the thorn is people-sized?

The obvious question: what comes to mind when you hear this phrase?
I don't know about you, but I immediately think of a rose.

Sooooooo

Think about roses, then. They've got thorns. All over them. Lots of people cut the thorns off. It's like the thorn is the enemy. Thorns hurt. We don't want one in our side. They're sharp, pointy--almost considered the downside of a bouquet of roses.

But why?

The rose doesn't feel the thorn.
In fact, the thorn protects the rose. It doesn't hurt it--not in the least. Without the thorn, maybe roses would all be eaten or something.

Why do we so often think of the thorn the antagonist and the rose the protagonist, rather than seeing them as a team?
Without the thorn, the rose would be weak.
And without the rose, a thorn is just a bother.

Alright now, here it comes:

Think about the thorns in your side.
The things that annoy you,
the people who drive you NUTS.

If they're not part of your team,
you're probably not the rose.