It's what seemingly makes Generation 4 more likable because of its oftentimes unspoken yet very emotional conveyances between characters. Spike and Twilight Sparkle are more like a mother and son whenever Spike is sad about something (Owls Well that Ends Well for starters), yet at the same time whenever one of them is feeling terrible about something that's going on (Say Lesson Zero for example) their relationship is treated just like that of brother and sister and it makes total sense in this regard. My only regret is the fact that despite this nature, why is it that Spike doesn't possess an Element of Harmony so he could become more centralized in big events or undertakings than just as a form of support? With his unique outlook and perspective he might have the potential to become a 7th element (NOTE: this is taken from the perspective that Season 4s first 2-parters, which I haven't seen only heard few discerning details about, never happened)