Since my dad bought my first bike when I was a kid I tended towards a leaned forward position. I don't mean hands on the drops position, just a more leaned forward one. Skip forwards for almost 30 years and I feel the same. Yet I keep hearing that the upright postion is supposed to be the more comfortable one. What am I missing here?
In general terms, a more forward position will mean that more body weight is supported by the arms, and that the back/spine is rotated forwards to a greater angle. The neck will also need to support the weight of the head at a more strenuous angle.
This will put more stress through joints in all of these areas and lead to faster fatigue of the supportive muscles, some people will find this less comfortable if they aren’t used to it.
Perhaps you have grown accustomed to your position because it’s how you’ve always ridden. Maybe your arms are used to supporting your body weight for example. This conditioning means you can be comfortable in a forward position for longer than average.
Riding out of your normal position probably means certain muscles have to work in unusual ways to support your body weight in a new position and they just aren’t used to it, so for you it is less comfortable.
Meanwhile, there will be other riders don’t have the same conditioning and experience things the other way around.
'Upright' is relative.
When you hear or read 'upright is more comfortable', that does not necessarily mean a near vertical dutch or cruiser bike riding position.
It's reasonable to say that most casual riders will find a torso angle of 25-45 degrees from vertical more comfortable than an aggressive position with the torso more than 45 degrees from vertical.