You aren’t supposed to do something about the confession.
If it was him that posted it, he didn’t do it expecting that you would respond.
He did it to make himself feel better about he fact that he can’t find it in himself to ask you out directly or face to face.
It was a way for him to relieve the stress or pressure that he may often feel whenever he thinks about the prospect of asking you out in real life.
His insecurities may stem from failed relationships from his past.
You may remind him of those relationships to some extent and he may worry that trying to make a move on you would only lead to a repetition of those failures.
If he has a history of short-lived and consistently botched relationships that have created doubt and anxiety in him, he can easily carry those insecurities into any future relationship that he has.
He worries about unwittingly making the same mistakes or brand new ones that he just can’t help.
A confident guy who is capable of talking to other girls can still get nervous and shy around a girl that he is romantically interested in.
In the same way that you get nervous and shy around him because you have developed feelings for him and feel like he is out of your league, he may also have the same sentiment.
He may have allowed himself to overthink the prospect of asking you out to the point where he has created stories in his mind about the person that you are.
This could lead to a development of complicated feelings that he doesn’t quite understand and is unable to get a handle on.
To add to that, he may see you as an attractive girl who is also out of his league.
Though he is capable of talking to other girls, he may not find them anywhere near as attractive as you. Hence, he may think of you as being out of his league and this makes him hesitant to make a move on you.
If you rarely get interested in anyone, it can easily translate into negative body language around guys.
Being that you are of this mindset, one would think that you would welcome the fact that guys never approach you.
However, you seemed bothered by this.
Wouldn’t the fact that guys don’t approach you be welcoming to you being that you rarely get interested in anyone?