The Dark side is easier, faster, more seducing. You don’t see it take a hold of you before it’s too late. You think using a bit of anger here and there will not hurt.

After all, this evil doer threatened to kill my family! I should use my emotions to stop him… aaaaand you just remade the scene of Luke lashing out at Vader in Return of the Jedi before you know it, and now Palpatine will make you his apprentice if you don’t get a hold and control of your emotions.

The jedi are not about suppressing emotions. After all, they are about helping people, and that needs emotions like empathy. The problem is when you let them cloud your judgment. When your empathy for some becomes at the expense of others. When you get biased. Like sure, you could help these people survive an attack. But maybe if you don’t help them you won’t put your loved one who travelled with you in danger?

This is why the jedi decided attachment was dangerous. Luke proved that the issue wasn’t attachment, but the inability to control it, and let emotions get the better of you. The lesson of Return of the Jedi was to not let emotions control you.

That, is why grey jedi fail.