Romeo believed he was putting an end to all the bad karma that began when he snuck into the Capulets' party. He pursued Juliet, encouraged the kiss and was the one who proposed the secretive marriage. He was also the reason for Mercutio's death, he killed Tybalt for revenge, and for all he knew he was the reason Juliet was dead. Because he had made all these poor decisions, his cousin and wife were now dead. With Juliet alive, with them as a couple, there was some hope for redemption and forgiveness and peace. But with her dead, his only punishment can be his own death. A death which will bring closure to the events he set in motion.
Juliet's mind was whirling with the 'What happened?'s and then the 'If...'s that people spend their life chasing. Of course she would choose to die; she was locked in a tomb with no escape, no food and no companion. Had she escaped from the tomb, she had no money, her father had already threatened to turn her out into the streets and she was a ruined woman. Not only was her death one of romance ('I can't live without the love of my life.') it was also one of necessity ('I have no other choice.')
Had she lived she would have had a hard life. She had dishonored her family and that would lead to her being wedded to whoever would have 'damaged merchandise'. Most probably she would have ended up with one of her father's business associates and been made to bear children to carry on his lineage. I think she would not have found love. She would have been miserable and mistreated and would have died just as lonely and desperate as she did in the tomb with Romeo.
Romeo was raging with many different emotions when he drank the poison. He laments the death of Tybalt, & questions why Juliet looks so alive still. Of course, it's because she is alive, but Romeo doesn't know that, & instead speculates that Death (in personified form) has kept her as his mistress. In fear of this, Romeo vows to stay with Juliet, and continues:
"Eyes, look your last!
Arms, take your last embrace! and, lips, O you
The doors of breath, seal with a righteous kiss
A dateless bargain to engrossing death!(115)
Come, bitter conduct; come, unsavoury guide!
Thou desperate pilot, now at once run on
The dashing rocks thy seasick weary bark!
Here's to my love! Drinks. O true apothecary!
Thy drugs are quick. Thus with a kiss I die."
In his last moment, he fills his senses, and steels himself against death. The fact that he dies "with a kiss" also suggests that he's still thinking of Juliet.
Juliet is thinking much along the same lines. She frets that Romoe has not left enough poison on his lips, & when she hears the watchmen approaching, she takes his dagger and stabs herself, saying "This is thy sheath; there rust, and let me die." Thus both think of each other as they die.
I'd like to think that Juliet would love again if she had lived. After all, she was only 13, and it would be difficult to believe that, being so incredibly young, she would not have found love again. Even Romeo loved before Juliet; it's quite feasible she could have loved after him. Of course, she may have found her life full of hardships, as her father had already disowned her and she would have to answer for her actions. So perhaps she would have entered a nunnery, or been forced to flee.