There's a lot of different themes in Romeo and Juliet and it'd do it a bit of a disservice to say it's about this one thing in specific.

But, Romeo's fickleness is a consistent through line.

Like, the first scene Romeo appears in is an argument between Romeo and his cousin Benvolio. Romeo is still pining over Rosalind rejecting him, and Benvolio tells Romeo that he'll thinks he'll fall for the next woman he meets.

"Be ruled by me, forget to think of her. By giving liberty unto thine eyes; Examine other beauties."

Romeo tells Benvolio that he'll never love another woman (note: this is dramatic irony, the ACT 1 prologue speech has already informed the audience he's going to fall in love with Juliet):

"The precious treasure of his eyesight lost: Show me a mistress that is passing fair, What doth her beauty serve, but as a note Where I may read who pass'd that passing fair? Farewell: thou canst not teach me to forget."

Romeo's closest friend is Mercutio. When he sees Romeo making a beeline for Juliet during a party they've snuck into he has this to say:

"Nay, I'll conjure too. Romeo! humours! madman! passion! lover! Appear thou in the likeness of a sigh: Speak but one rhyme, and I am satisfied; Cry but 'Ay me!' pronounce but 'love' and 'dove;' Speak to my gossip Venus one fair word, One nick-name for her purblind son and heir, Young Adam Cupid, he that shot so trim, When King Cophetua loved the beggar-maid! He heareth not, he stirreth not, he moveth not; The ape is dead, and I must conjure him. I conjure thee by Rosaline's bright eyes, By her high forehead and her scarlet lip, By her fine foot, straight leg and quivering thigh And the demesnes that there adjacent lie, That in thy likeness thou appear to us!"

And, like, other than those two, the only person Romeo confides in is Friar Laurence (who I quoted in my previous comment).

So, both the events of the play and exposition from characters closest to him all take pains to highlight Romeo's tendency to fall into obsessive infatuation, only to immediately forget about them the moment the next object of his lust crosses his view.

That's not to completely invalidate interpretations of Romeo and Juliet as a tragic love story, just that it interpreting solely as that requires flattening out a lot of the wider themes.

Sorry for the text wall.