Can you solve this riddle involving alphabets?

You need to find the answer to the riddle within the question.

“I am the beginning of sorrow and the ending of sickness. You cannot express happiness without me. I am in the middle of crosses. I am always in risk but yet not in danger. You may find me in the sun but not in the moon.”

“Can you tell me what am I?”

Answer of this riddle: The letter S.