Why do you think Voldemort wanted to be immortal?

His mother died and he did not want to end up like her, weak. He saw death as a weakness, and that is also why he thought she could not have been the witch in the family. He was also repulsed by the idea that his father was alive and not there for him.

So he could be in power forever and if anyone opposed him he could fight them with them having no chance of defeating him. Which of course posed as a failure in the end anyway.

I'm pretty sure his greatest fear was death.

He was afraid of death - like most of us.

But unlike most of us, he had not known love, the force that is stronger than death, through the persistence of soul. Hence his little consideration for his own soul.

Just my 2 knuts.

He was afraid to die, bro.

his fear was death. and he wanted to be the most powerful

Of course he wanted to be immortal. He saw death as ur last defeat and proof that u were weak. he feared what normal people love , fear and take for granted.

So that he would NEVER die. ;)
Think about it, no matter how rich, wise or powerful a person might be, death is the only thing they can never escape. Just like the good o'l danse macabre. Being immortal would mean doing something noone has ever managed to do before.

Moreover, and in my opinion more importantly, the Dark Lord was afraid of death, aren't we all after all? I know I would like to be immortal even 'only' for the sake of not having to grow old and die.

Not to mention the incredible amount and diversity of things he could learn and experience should he live forever.

As someone else has already said: his mother died and he has not ever known her, his father abandoned him and he grew up in an orphanage, surrounded by muggles, strange things happen while he was unaware and did not understand what was wrong. He has never known love, the only thing that can "defeat" or at least make it more "gentle" death. I believe that in a sense he is jealous of Harry, his friends, Dumbledore and everyone else because they love and live in harmony, addressing the difficulties together and always helping, and so he tries to reunite the people (Death Eaters ) because he wants to be loved and respected. He moves from the dark side because he wants to be somebody, be considered and later feared by all those who love and live in harmony ... he just wants others to understand how he feels, what he lived and what they could not live (parents, love, friendship). So in other words I think he's jealous, wants to be understood, to be loved and at the same time he wants that others pass what he has passed in his childhood.
This is just my thought .. may not be as it.