Originally Posted by Tchiowa

An open window shade will also change the ambient light level and make it hard for people to see their IFE monitors. And some people can't sleep with a giant elastic band over their eyes. You, on the other hand, have lights you can use to read.

So windows shades closed you can read, others can sleep, others can see their IFE. In general, everyone can enjoy the flight the way they want to.

Window shades up you can read but you will have a negative impact on others.

On all TPAC flights I've been on the FAs tell you to close your shades. Common courtesy says you should.