The reason why Darth Vader was constantly making changes to his suit in Star Wars is a little tricky since there’s a difference between the legends canon and the current canon (thanks Disney). In the legends canon he didn’t make that many alterations to his suit despite the fact that it caused him a great amount of pain. You have to remember that when he was put into the suit he was burned beyond all recognition and would have died eventually had the Emperor not found him. By hardwiring him into the suit Palpatine made sure that Vader would survive, but he also ensured that Vader would be his servant for as long as he needed him. The suit was all that kept him alive since anything that was human in him slowly died out over the years, replaced by machines and consumed by pain, and anger. By not changing his suit save for a few alterations here and there, Vader accepted the pain it brought him as it made him stronger in the Dark side as his rage continued to take over. This made him a more efficient killer and far more ruthless when it came to dealing with his enemies.
In the current canon that can be followed from the third episode to the sixth Vader managed to make many upgrades that allowed him free range of movement and a much quicker response time from his cybernetic parts. The part of him that refuses to die, the skills learned by Anakin Skywalker, enabled Vader to work with Imperial scientists in an effort to modify his suit so that he could be a more efficient and ultimately more lethal killing machine. These modifications made it possible for his body to continue even though it was still dying as the years went by, and to replace parts and pieces of himself that were eventually of no use any longer. In the end Obi-Wan was right, he was far more machine than man.
Look at it this way. If you ask an amputee in the real world what it’s like to use a prosthetic limb they might tell you that they get along just fine, but it’s nothing like the real thing. It’s no different in the Star Wars universe except for the fact that their limbs tend to be a lot more functional thanks to science fiction. Somehow the nerve endings and connectors between the cybernetic limbs and the flesh of the person using them are completely capable of bonding and responding to one another. After his battle with Obi-Wan on Mustafar though Vader was without three of his limbs, and needed to be given new legs and an arm to function. He also needed a breathing apparatus since his lungs were scorched, and a life support system since he would have died otherwise. Vader was in pretty bad shape when Obi-Wan left him.
And yet not long after Vader arose with an intent to kill every Jedi he could, filled with hate and malice that his suit only continued to stoke as initially it was quite painful to use. It’s not much of a surprise that being a skilled tinkerer that he would work to make it more bearable though.