I just wanted to say anyone who thinks leitmotifs are lazy is an idiot. They're great for drawing parallels between situations, characters, themes, moods. I think the constant musical callbacks are better done in Undertale than any other game I've played - and the fact I remember most of the music in Undertale (as opposed to say, Dark Souls, a game I love but I can only really tell you what Firelink Shrine, the O&S theme and Gwyn's theme sound like) is because of the leitmotifs.
Let me give you one of my favorite examples: Heartache and ASGORE. They're essentially the same song remixed to fit the situation,
but it makes perfect sense to use these tracks in this manner. The first version is more barebones and melancholy, befitting the exiled queen who really doesn't want to hurt you (and if you're playing pacifist, you don't want to hurt her either, making it even more fitting). ASGORE, though? It's both more intense - suiting a battle to the death with the most powerful
* monster in the underground - and (to me at least) royal sounding, which works because he's the reigning king.
...Track 71, Undertale, is still the best track though. I really like how that scene plays out, and I've been thinking about something. Near the end of the 'encounters', the monsters ask Frisk 'You're going to be free soon. Aren't you happy?' This is interesting to me because in the true pacifist ending, Asriel flat out tells you that all the monsters love Frisk... so when they're asking Frisk that, do they believe s/he is going to go with them to the surface world when another human arrives for Asgore to kill (keep in mind, that only Sans, Undyne, Alphys, Mettaton, Toriel, Flowey, and Asgore seem to know what humans look like; Papyrus and Monster Kid are completely clueless, as are all the shopkeepers except I think maybe Gerson? Therefore it's reasonable to assume the majority of monsters you encounter don't actually realize you're human either), or do they believe that s/he is going to willingly sacrifice themselves to Asgore to free the monsters?