Intense comes from Latin intensus meaning 'stretched tight' — something pulled taut like a bowstring. The same root produced intend, tent, attention, and tender.
Of extreme force, degree, or strength; deeply felt or earnest.
From Latin intensus, past participle of intendere meaning 'to stretch out, to strain, to direct', from in- 'into, towards' + tendere 'to stretch'. Something intense was originally stretched tight, like a bowstring pulled taut. The metaphor extended naturally: a person who is intense is stretched to their limit, fully directed towards something. The same root gives us intend (to stretch the mind towards), attention (stretching towards), and tent (a stretched cloth). Key
Intense, intend, tent, attention, extend, pretend, and tender all share the Latin root tendere meaning 'to stretch'. A tent is stretched fabric. To attend is to stretch your mind towards something. To pretend is to stretch something forward as a false front. An intense person is simply someone stretched to their limit.