In an Atwood machine where we assume the string is massless, I understand that the tension in the string is constant throughout. However, I'm having a little trouble imagining the forces on the pulley itself. I'm not sure why the pulley feels $2T$ force downwards, where $T$ is the tension in the string.

[![enter image description here](https://i.stack.imgur.com/tw2uU.png)](https://i.stack.imgur.com/tw2uU.png)