pulseaudio doesn't work permanently

I use pulseaudio on Ubuntu.After I start pulseaudio daemon using pulseaudio --start command, it starts working well.But it doesn't work permanently. It turns off very soon.So I have to retype pulseaudio --start every time I want to play music.

Additional information

When I check the list of processes related to pulseaudio, I find it working as a system daemon:

$ps -alx | grep pulse

~ UID ~ COMMAND

~ 108 ~ pulseaudio --system

~ 998 ~ pulseaudio --system

I can't understand why processes work as system in other user.