One of my biggest frustrations during my flute-playing youth was the difficulty I had making crescendos. It had nothing to do with the maximum volume of the sound I could make, because I never had any trouble playing loudly when I needed to, but getting there was always a problem for me.
I always thought of it as a personality thing, reflecting the frustration I would have with not being able to assert myself in certain situations. I associated my crescendo problem with dreams that I had where I could not be heard if I tried to say something, or dreams where I was physically not able to move fast enough to get away from an element of danger. In waking life I associated it with, not being smart enough, not being tall enough, not being clever enough in social situations, or feeling tongue-tied under pressure.
Now I see the crescendo problem in some of my teenage students. Some don't seem to have a problem with making crescendos at all, but the ones with a teenage psychological mindset similar to the mindset I lived with for those fragile years, become musically frustrated in the same ways I became frustrated.
I take special glee in slaying the dragons that plagued me in my youth by teaching these kids technical ways of making their crescendos. It is so much easier to use technical means when the technique is on the outside. Everything concerning sound on the flute is located inside the body, and it took a long time before I found a teacher who had the ability to explain the way sound production worked on the flute (that teacher was Keith Underwood). Learning how to make a crescendo really improved my musical life, and by extension, it improved my whole life.
On the violin and viola much of the equipment is on the outside. There are many ways of making crescendos using different parts of the bow arm, using varying amounts of bow in various ways, moving towards the bridge and/or to a more resonant place on the instrument, where a stronger finger can shoulder the burden of going from mezzo-forte to forte, and even onward to fortissimo.