What are 3 reasons that show Romeo and Juliet aren't actually in love with each other?

To answer this question, it's first important to define what being in love means. Often it is defined as really knowing the other person, the beloved. Love, in other words, needs time to fully develop. Second, real love is not based on fantasy or illusion. It means loving the whole person, flaws and all, and being able to forgive the flaws and appreciate the strong points the other person possesses. It is not putting the beloved on a pedestal as a god or goddess figure. Third, love puts the needs of the beloved first.
With this definition in mind, we can see that Romeo and Juliet are infatuated and physically attracted to one another but not in love. They barely have any time to get to know each other before they are each dead. They are married within twenty-four hours of meeting one another, and after their wedding night, during the rest of the very few days they are married, they are separated. They don't know each other.
Second, Romeo, especially, puts Juliet on a pedestal, seeing her as a goddess-like figure more beautiful and brilliant than all the stars in the sky. They each idealize the other, and neither believes that life can go on without the other. This is overwrought fantasy love, not real love.
Finally, Romeo is not able to put Juliet ahead of competing relationships. It's hard to blame him for this, as he scarcely knows her. Nevertheless, he loves Mercutio so much, arguably more than Juliet—which would make sense, since they have been friends for a long time—that he jeopardizes his relationship with Juliet to avenge Mercutio's death. He also impulsively kills himself when he thinks Juliet is dead, putting his own grief ahead of thinking that his beloved would, of all things, not want him to die.
The two young lovers are infatuated, and over time their relationship might have developed into genuine love, but at their death it is a fantasy love.
https://conscious-transitions.com/real-love-versus-infatuation/

https://www.huffpost.com/entry/what-does-it-mean-to-be-i_b_3531407