At my stage (student, with around ten hours) it's the right kind of stress.
But it's lots more. My father flew and I used to go up with him in his Cessna 172 as a kid. He could fly! I never felt nervous with him, i was a kid and dad can do everything.
I last flew with him as an adult, around 1990-something, and loved it...felt good to go up again with him.
He has passed (2008) and I had always wanted to learn to fly, but lived far from my dad, never had enough money to pay for lessons. Eventually forgot about that dream. Then at 59 realized I did have the means if I wanted to do it. Kick myself for not realizing it earlier, but I started.
First flight with my CFI taking off, as soon as we were up in the air on climb out I was grinning from ear to ear and happy as I've been for a long time. I did pretty good also at basic level flight, some banks, etc.
There is just something so RIGHT about it, at the same time as it feels like we are doing something we really aren't supposed to be able to do, lacking actual wings on our bodies...yet we can do it. The sight from up in a plane never ceases to make me smile and feel wonderful. Even on a jet liner, I wonder about all those people ignoring that incredible view (if they were on a mountain looking down they'd all be oh'ing and ah'ing...but on a passenger plane just go right to to their iPads, totally ignoring all that out there, and when else do you get above clouds?? How cool is that?) but in a small plane it's somehow even more there.
Now it's stress, I think my current CFI wants to throw lots at me to keep me able to handle stress with a lot of maneuvers, but I notice when my hand is tight on the stick or yoke, and remember even though I'm up there to learn to be a pilot, if HAVE to get some "alone" look-at-that time, and that enjoyment. Less of it right now because of training.
I really don't know, but I don't need to know, why...just it feels so right, and so wrong at the same time. And the views. And being able to learn how to actually pilot, and navigate in a small plane...I don't know why everyone that could afford it isn't doing it.