Gaster is a sick, twisted, intelligent and sophisticated monster being, cruel enough to hold you captive and extracte the powerful determination your soul held, along with doing other sick soul experiences on you to see how much humans can handle, how much pain tolerance you have, how much determination your allowed to hold, what happens if different soul powers combined, and since your DT (determination) was so powerful and it regenerated quickly, you were useful. Your actions amused Gaster to.