Both are fed by the same source of life, the Jordan River. The Sea of Galilee is full of life. The area around the Galilee supports an abundance of wildlife. I am told by friends that it is absolutely a beautiful area. In contrast, the Dead Sea is lifeless.
Nothing lives in the sea and few living organisms are found around the Dead Sea.
If you look at the map, the answer is quite obvious. The Sea of Galilee takes what it needs from the Jordan River, multiplies the gifts and then passes the water on downstream. The Dead Sea takes water and hoards the water, never passing any water on. The Dead Sea only consumes. The water in the Dead Sea evaporates, leaving behind salt, thus making the Sea lifeless.