Cycling is an activity where strength and experience increase together - until you are over the hill

Descending at silly speeds, especially when following somebody else too closely are both signs of inexperience and IMO in these juvenile showing off situations tend to be to compensate for being unable to keep up on climbs. Making a big effort to pass somebody then fading immediately is about as crass as it gets. Some posters are miffed by strangers sitting on their wheel but I get irritated by this competitive showing off (especially if it's pouring down and the person who thinks it's clever to be in front has no mudguards.

) I don't think any of that is what the OP was concerned with.
Somebody above (BigT?) commented how it's not always easy to overtake in traffic. It seems to me that when commuting - as thread title - most riders are riding towards the upper end of their ability in terms of speed: not racing, but pressing on. It seems inevitable to me, therefore, that in the normal course of things, stronger riders are going to be catching up with those going a bit slower. In traffic, overtaking safely is not always easy. I remember a personal apology published in the CTC mag ten or twelve years ago from the then CTC director, Kevin Mayne. I cannot remember the details but he had become involved in some sort of misunderstanding when he had caught a female rider who had taken umbrage for some reason and having seen the orange sticker on has back mudguard shouted something like "Call yourself CTC?" as he rode off down the road. He seemed mortified by the complete misunderstanding.
I'd like to think that the apparent increase in riders knowing how to shelter behind another is generally a good thing. I like to think there's a strong sense of mutuality among cyclists: I've always been grateful for help given by other riders and I've been pleased when I've been able to help others. OTOH, some posters seem to suggest that there are strong/ experienced riders sitting on wheels as though they were policing an attack in a road race. Is this really as frequent as some seem to suggest?