Romeo's greatest tragic flaw is his immaturity, which in turn means that he makes irrational choices, ignores good advice, and, crucially, does not know how to love well or wisely.
At the beginning of the play Romeo is madly in love with a girl called Rosaline. He proclaims that Roasline is "rich in beauty" and a "precious treasure," and he complains that, because Rosaline does not reciprocate his love, he "live(s) dead" or, in other words, might as well be dead. However, as soon as Romeo sees Juliet, at the Capulet party, he forgets all about Rosaline and proclaims the he is now madly in love with Juliet. The only difference between Rosaline and Juliet seems to be that the latter reciprocates Romeo's love, whereas the former doesn't. When Romeo runs to see Friar Laurence, the morning after meeting Juliet, Friar Laurence asks Romeo, "Is Rosaline, whom thou didst love so dear, / So soon forsaken?" The intensity of Romeo's seeming love for Rosaline and then for Juliet and the speed with which he falls out of love with one and in love with the other all point to Romeo's immaturity. Romeo does not really understand what love is. He is still a child with immature ideas of love. He thinks that love must be instant, all-consuming, and obsessive. Later in the play, it is this immature understanding of love which makes him unable to, in the words of Friar Laurence, "love moderately."
Friar Laurence gives Romeo lots of good advice. As noted above, he tells Romeo to "love moderately," and he also warns Romeo that "violent delights have violent ends," meaning that those who love too violently, or too intensely, like Romeo, will inevitably suffer the violent, fatal consequences of that too intense love. Romeo though is too immature to heed this good advice. Instead of trying to speak to Juliet's parents, and perhaps brokering a peace between the two families, he seems to revel in the illicit nature of the relationship. At the end of the play, he decides to kill himself, when a more moderate outlook might have resulted in him and Juliet being together.