single lever faucet won't stay open

As you can see in the title, the faucet won’t stay open.

I turn it on part way and after a moment it starts to close on it’s own.

Is something loose? Do you have any ideas?

Depends on the brand of faucet, on Peerless you can remove the lever and the “cap” and there will be a plastic fitting with notches that you can turn to tighten, others are la lttle different but there usually is a way to tighten them. A Moen has a nut you can tighten, doing either will most likely be just a temporary repair, replacement cartridges are available for almost every brand.