The sun, the earth, the water and whole of nature keep giving without any conditions. It does not expect anything back.
Giving is inherent in nature, and so it is supposed to be in humans. When we give unconditionally we become one with nature and feel harmony with our surroundings.
Can you imagine the sun saying, " I will give my light if you pay me x dollars or only if you return the favor." It is amazing how many things we take for granted, that have just been given to us, including our incredible eyes, brain and limbs that perform miraculously every single day.
So instead of asking the question, "Why should I give?" let us ask "Why not?"