I often let my toddler son ride on the diesel tractor all by himself
Why would you let him ride the tractor unsupervised?
The tractor is perfectly safe. I would feel so sad if I didn't let him do the things his way.
A tractor is not a toy, and you could easily endanger a toddler's life so I do not recommend that you leave your toddler in there anymore. Try to just hire a nanny that can look after your toddler, instead of you neglecting your parenting duties by letting him ride a tractor.
You don't get it. I'm his dad and it's my job to make him happy.
You are putting him in danger though! You should never leave a child that young unattended, especially on a huge piece of farm equipment like a tractor.