Startup Applications do not stick, they get removed if I add them

As the user, you can set more startup applications from
gnome-session-properties

(Application Menu / System tools / Preferences / Startup Applications)

I added two new applications,

however, I noticed that they did not start on the next startup.

This was because they were not added permanently to the Startup applications.

and this was because my ~/.config/autostart forlder's write permission owner was the root.

I changed this with sudo chown -R myuser:myuser ~/.config/autostart

Probably running sudo gnome-session-properties would also have been enough, but I did not try.