why can your teammate destruct your commander?

How is that your commander?
Your team begin with a set of commander, they belong to everybody as every other units and building.
Play with people you know if you want to put arbitrary limitation, or wait for an 'Alliance' game mode.

Make it toggleable ingame. Also, disable teammates destructing your units.

If i see someone taking my commander for a joyride, I'd like to lock my commander. Otherwise they can use it

100% agree with that ticket. Even in replays I've seen teams screwed over because players get mixed up with commanders, messing up build queues for each other, and giving bad orders to each other in combat and other tense moments. A simple check box to enable or disable sharing would be all that's needed to stop 90% of the problems, and still allow full sharing.

Additionally, from a lore/immersion perspective, a Commander is not just some mundane expendable unit for other commanders to mind control about. The Commander is supposed to be your in game avatar during play. It's the physical body of a sentient being. Even if they are equipped with, lets say, a system to allow sharing of their functions to other trusted commanders, you would think they would have the end say in what their own body does.

To be frank, if you lose your commander in a shared army game, really you should lose control over the game, just the same as in a normal team/alliance game where assassination mode is the norm. Because YOU have died. It's a shared army game, not a hivemind game. Although I'd find it entirely reasonable to include a game lobby option to allow continued control after death of course, just like the difference between classic Assassination play in TA or SupCom, and the Supremacy or Sandbox modes.

It might sound harsh to some, but I don't feel like commanders should be thrown around like disposable "Commander Bombs" every mid game anyway.

It's not a unit or a bomb on legs. That's supposed to be you.

Gameplay reason: Army play is meant for coordinated teams. And not really anything else. Alliances are multiple Commanders working together.

Rational reason: All of the Commanders that are part of an army form a hivemind. A decision to destroy one Commander is a decision on the parts of all the Commanders. They are functionally sacrificing part of their brains to try and stave off an attack or gain an advantage.

Lore reason: Well, we're talking about immersion an an RTS, which is like talking about drinking butter with your salad.