Especially with a couple of different races like this.
Can you imagine.
They start out as a sort of hierarchical relations at first (parent-child, master-servant, etc)
Grow closer, form deep bonds,whether romantic (or even sexual) or not.
In time, the one with longer life span will likely grow enough for them to look 'natural' side by side.
Then as time goes on, the one with shorter life span will slowly grow old, and DIE.
The one that remain will grieve, but then what next?
Will they keep living on, support by the memories of those days, or will they fall into despair, losing the one that gave light to their days, warmth to their nights?
If they keep living on, will the two eventually reunite through a cycle of reincarnation, starting their relations again with the role reverse?
etc etc
Basically, anyone who want example of this, just look up
"Let's Meet at The Witches' Gathering" (Japanese: #魔女集会で会いましょう ) images.
Note that your reactions may range from 'aww that's cute' '*screaming fangirl*' to 'no no no plz not like this' to 'whyyyyyyyyyyyyy god, whyyyyy'