Note: contains major spoilers for “The Mortal Instruments” and “The Infernal Devices” series.
It's a well-established theme in paranormal romance and romantic urban fantasy that true love should be forever. If one lover is mortal and the other isn't, it has to be remedied in some way: this can be as simple as a vampire “turning” his beloved, or as mind boggling as some of J.R. Ward's solutions, which usually involve divine intervention. A lover who can't or won't be turned is generally tragic backstory.
Cassandra Clare's interconnected YA Shadowhunters books are unusual in exploring relationships between mortals and immortals as a viable option, albeit not one without problems.
In The Mortal Instruments series, mortal teenager Alec falls in love with Magnus Bane, an immortal warlock who's over three centuries old. The bliss of requited love is marred by Alec's jealousy over Magnus's many previous relationships, and by his worry about growing old while Magnus stays youthful—shades of Bella in the Twilight series, but this story goes in a very different direction. With no desire to be immortal himself, Alec contemplates a spell to take away Magnus's immortality, and he realizes how wrong and foolish this would be just a little too late—Magnus finds out about the plan and dumps him, breaking both their hearts.