Answer:
They are passionate, but their love transgresses social boundaries.
Step-by-step explanation:
From the beginning of the story (and their lives), Catherine and Heathcliff identify with each other. Catherine famously states that she "is Heathcliff," and they seem destined to be together. However, Heathcliff is of low birth, and Catherine grows concerned with the possibility that marrying him might affect her social status. This leads to tragedy for both characters, due to the fact that their passionate love transgresses social boundaries, and that the society they live in is not one which cannot accept such turbulent and carefree attachment.