More input is always welcome, as no answer should ever be taken as final. In the meantime, as this has popped up before, I commend to your attention these previous explorations as to why people of varying classes would sign up (or be signed up) for the armies:

• u/Rittermeister explores in these two posts the why of things;
• u/MI13 examines who generally makes up Medieval armies;
• and u/Goiyon has a very comprehensive overview on Dutch armies of the period.