William Shakespeare's "Romeo and Juliet" is generally regarded as a tragedy because it features dramatic and devastating events when the two main protagonists die at the end. It doesn't, however, fit the conventional mode of Greek tragedies.
"Romeo and Juliet" is considered a love tragedy because Romeo and Juliet died due to a sequence of dramatic and distressing acts related to their love for each other. The play has elements of comedy, though, which serve to distinguish it from more traditional Greek tragedies. Also, a conventional literary tragedy features a dramatic death of a high-ranking character, not a story's protagonists.