He could be afraid of dark, water, sun, needles, room.
He can have a trauma of being: alone, fat (he used to drink a lot of water), left in sun for too long.
Other cacti used to bully him.
He injured something with his needles in the past.
He is afraid of heights and he live on the windowsill.
He is claustrophobic and he get packed to a small see-through box for the time he is in shop.
He is afraid of getting any sand in his eyes.