Knuckles is tough and stubborn, and is willing to attack anyone who messes with the Master Emerald or the harmony of the Floating Island. Guarding the Emerald, though, leaves Knuckles to lead a very lonely life. He often has to choose between friends and the Emerald, and it's obvious which he chooses.
A lot of Sonic fans quickly tag Knuckles as antisocial without investigating the reasons why. There are a few, and I personally believe he's not a loner by choice. This can be quickly supported by the fact that he befriended Sonic and Tails in Sonic & Knuckles after he learned of Robotnik's deception, and continues to fight on their side time and time again. I wonder, though. Does he really become friends with Sonic, or does he unite with them because they share a common goal?
I think it's hard for Knuckles to get close to anyone for two main reasons. The first, and probably most obvious is that he doesn't know how. He doesn't understand the nature of others, proven again and again by his quick gullibility to Doctor Robotnik, and why should he? He's been alone for as long as he can remember, the last of his species, guarding the Master Emerald. It's all that he knows.
The second reason Knuckles doesn't get close to anyone is because he's scared to. It's my inferred theory, but I personally think it has substance. Knuckles is well aware of his fate and his calling. He doesn't get close to others because he doesn't want to get used to companionship. He knows he'll eventually have to return to the Floating Island and live his life alone.
Is there hope for him? Probably. Knuckles becomes more and more sociable as the story goes on because he gets used to interacting with others. It's easy to see the strides he's made when he jokes around with Sonic. Does he need hope? No. I think Knuckles is perfectly content with the life he's leading. It may be a lonely one, but it's one that he has accepted.