Grimlock, and to a lesser extent, the Dinobots as a whole, break from the traditional Autobot mindset. Grimlock values strength over wisdom, which usually puts him at odds with the wise Optimus Prime, who would rather avoid a fight when necessary.
Normally this might make him a villain, but despite his crude nature Grimlock follows a personal code. Though he appears to feel a bit of contempt for weakness, he uses his strength to help others, and his true hatred is reserved for those who use their strength for cruel purposes. Though he doesn’t exactly fit with the Autobots’ beliefs, he still appears more comfortable with them than with the lying, cheating, stealing, and generally jerky Decepticons.
It’s a good thing for the Autobots. Since they all share Grimlock’s belief in “might makes right”, none of the Dinobots are really philosophically in tune with the Autobots as a whole, but as their strongest member, Grimlock keeps them in line.
Also, like the rest of the Dinobots, Grimlock no speak good.