Partially why I'm so enamored with the song is how the song goes through all these little, unexpected shifts, form the gospel-influenced, piano and organ intro to how the strings and horns drop in at the first chorus and especially to how the background singers slide in during the back end of the chorus to lay some doo wop sweetness on you. On some songs, you might have a favorite part you wait to hear, again and again, but on this one? I have at least 3 or 4 of those.
One last thought: The Impressions obviously recorded a ton of incredible ballads, most of which were about crushing heartbreak yet many of them sound...happy. If you don't listen that closely to the lyrics in "My Deceiving Heart" (ok, and ignore the title while you're at it), it feels more uplifting and optimistic than a song about someone who just found out that his lover's been cheating on his ass all this time. (Of course, no song from their repertoire is as "joyous in feel yet devastating in message" as another recording from this same album: "Seven Years.") It's not that this is unique to The Impressions but I also have a hard time thinking of any other artist that turned out so many sad songs that didn't actually sound sad to me.
(For best results, listen with headphones)