After the heartrending finale in which Monica and Max's mom have to leave Max forever, Monica appears right smack dab out of nowhere in the epilogue. She used a starglass to illegally travel to the past to see Max again. Starglasses work by transporting the holder to the single most important point in time-space that they wish to be in. Since she appears in the middle of a dungeon right where Max is, it's obvious that Max is the most important thing to Monica. Awwwwww... And since she's now stranded there, that means she and Max get to spend as much time together as they want!
Any of the flashbacks involving young Max and his mother, due to how sincere they are.