Picture a line that heat is fine below and dangerous above. Warming something up is useful all the way to that line: the engine runs, the sauce , the body works harder. Cross it and the very same heat starts to break things. Over- in this word is not simply a lot; it marks the moment the line is crossed, which is why the word almost always arrives as a problem or a warning rather than as a description of temperature.
From the word’s history
The source records overheat from Middle English overheten, analysed on the surface as over- plus heat, which is exactly the picture of heat taken past its limit.
本批評證記錄 overheat 來自中古英文 overheten，構詞上就是 over-（過頭）加上 heat（熱），正好對應「熱越過那條線」的畫面。
Wiktionary entry for overheat supplied in this packet (kaikki:3f6f69a190036ae4:1368074859:18886), etymology_text: From Middle English overheten ... By surface analysis, over- + heat.
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