Romeo is a fairly typical teenage young man.
He is certainly impulsive. He acts before he thinks. This is evidenced most powerfully and tragically when he hears that Juliet is dead. His immediate reaction is to return to Verona so that he can commit suicide in her tomb. This would seem to be rather impulsive. Had he taken time to think about and evaluate the situation, he would have sought out Friar Laurence to find out what happened.
In the beginning, he would appear to be superficial. In Act I, scene 1, he is lovesick for Rosaline. He only has to see Juliet in Act I, scene 5 to fall out of love with her and into love with Juliet. On the the surface, this would appear superficial but if one were to look more deeply something extraordinary happens when they speak. Romeo begins a sonnet which becomes an extended metaphor and Juliet continues it. Shakespeare is definitely telling us something about these two young people. They complete each other; they are part of the same whole.
It might seem to be selfish that he marries Juliet and puts her into danger, but it must be remembered that Juliet is the one who suggests marriage in Act II, scene 2.
Romeo is a much more complex character than he would seem. Where strong emotions are concerned most people don't behave rationally, so why should a teenage Romeo?
As the previous poster pointed out, he certainly doesn't consider the things that Juliet will have to deal with if they get married, but this is somewhat easier to understand because both of them will have to deal with plenty of hardship when they choose to marry the child of their sworn enemy.
But of course he also demonstrates selfishness when he ignores the danger signs repeatedly in order to continue to pursue Juliet despite the increasing tension and the increasing danger to his brothers and cousins.
Romeo acts selfish when he doesnt take into consideration what Juliet will be put through if she goes about marrying him with the Friar. He doesnt realize that her father and mother could give her severe punishment for that action. You would think if Romeo really truly cared about her that he wouldnt make her do such a thing. Even though Juliet wanted to, Romeo should have taken a different course of going about it.
Alas the lovwhose view is muffled still, should without eyes see pathways to his will. Where shall we dine?-O What fray is this?
Act 1. Sc.1 165
Here he demonstrates that he has an implusive and whimiscal tendency as he talks about Rosaline, he qiuckly moves on to another subject like "where shall we dine?" then straight to asking about the town square brawl.