I am considered ‘intense’ so I can try to answer your question - although your definition of intense may be different than mine.
I am intense because my feelings and emotions range from 1 to 10, from the depths of despair to ecstasy. For some people their emotions seem to range from a lukewarm 4 to a lukewarm 7. I envy them for not feeling agony but also feel sorry for them for not feeling intense beauty, or intense pleasure.
I am very sensitive, everything touches me. I could say that I am overly sensitive so sometimes I may get upset or sad about things that other people can easily ignore, but I can’t help how I feel. (For example if I read an article about animal cruelty I’m in absolute bits - but this does come from complex childhood trauma about lots of animal neglect and cruelty)
I am intense in my friendships, I have few close friendships but I don’t really have acquaintances, I don’t see the point of anything lukewarm.
I have high standards, I am 100% reliable and I expect the same. Same goes with other traits such as honesty, generosity etc. I am intensely loyal, intensely passionate, but also intensely furious if betrayed.
i don’t know why I am ‘intense’. I just am. I think its more nature than nurture. Actually, I don’t know, I know my feelings were ignored when I was little, as if I didn’t matter, so probably all those upsets and frustrations intensified leading to my current intense personality. I sometimes wish I was more casual and cared less about everything but most of the times I am glad I am the way I am - in my mind the opposite of intensity is a ‘lukewarm’ and ‘can’t be bothered’ attitude.