Teiresias uses a simile to promise Odysseus THAT HE WILL HAVE A HAPPY DEATH. Simile is a figure of speech that is used to compare one thing to another one in order to make the description very emphatic. Teiresias used simile here to tell Odysseus that he will die happy at a good old age.
The main difference between his love for Rosaline and Juliet is that he did not love Rosaline for anything but her beautiful looks while he fell in love with Juliet for both her looks and personality.
The first time he meets Juliet he realizes this and expresses it on a sonnet by stating ''Did my hear love till now? Forswear it, sight''.
His feelings are manifested by his actions, the moment he realizes his love for Juliet he decides to marry her despite the circumstances.
At the beggining of the play Benvolio states that Romeo's love for Rosaline is mainly a puppy love; Then later on, we discover that Romeo's love for Juliet is described as the one true love, they then become starcrossed lovers.
Hope this helps!
