The "Love Theme from Titanic" is a song that uses harmony in thirds. Creating harmony using thirds in a major key and thirds in reverse, called sixths, tend to sound good in music. The song is in the key of E major and moves to Ab major in the final chorus. Harmony adds variation to sections. The harmonized notes in verse 2 melody "Love was when I loved you..." is in thirds - E(G#) D#(F#) F#(A) G#(B) F#(A). In reverse as sixths would be - G#(E) F#(D#) A(F#) B(G#) A(F#).

Another point to note is the leap resolution. The melody does not leap more than an octave, for example (B3 - B4), and after the octave leap it changes direction. This idea is said to help in producing singable melodies, since larger leaps moving one after the other can be difficult to sing. I also mention the vocal registers (chest, head) and vocal techniques (belt, falsetto).

P.S. The many cuts in editing is to avoid copyright infringement.

https://youtu.be/EH03o3S3cn8