Probably the emotional requirement. He kept himself pretty calm, and what little anger he displayed wasn't anywhere near enough to go SSJ.
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Yes, but if pushed could he go SSJ? What would that even look like?
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I think if pushed, he might be able to, but even Vegeta, known to be angrier and more desperate for SSJ than anyone, took a long time to reach that state. I don't think Cell would reach it spontaneously. I think he'd have to really work to attain it. Maybe his skin changes hues to a different shade of green, or even another color altogether.
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Would his head points get longer?