为什么我用 systemd 创建的服务全部都会自动关闭？

我的操作系统是 Ubuntu 16.04.02LTS 是从 14.04 升上去的 比如 squid ： /usr/local/squid/sbin/squid -s 是后台启动 /usr/local/squid/sbin/squid -k shutdown 是关闭

我新增了一个文件：/lib/systemd/system/squid.service

[Unit]
Description=squid , a HTTP(S) proxy server
Documentation=https://blog.ruiruige1991.xyz
After=network.target  

[Service]
Type=simple
ExecStart=/usr/local/squid/sbin/squid -s
ExecStop=/usr/local/squid/sbin/squid -k shutdown
ExecReload=/usr/local/squid/sbin/squid -k reconfigure

然后每次执行systemctl start squid.service，发现都起不来

执行journald -xe 发现有以下日志:

Mar 06 16:04:41 NyaVM-VPS-updwugzl systemd[1]: Started squid , a HTTP(S) proxy server.
-- Subject: Unit squid.service has finished start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit squid.service has finished starting up.
-- 
-- The start-up result is done.
Mar 06 16:04:41 NyaVM-VPS-updwugzl squid[21867]: Squid Parent: will start 1 kids
Mar 06 16:04:41 NyaVM-VPS-updwugzl squid[21867]: Squid Parent: (squid-1) process 21870 started
Mar 06 16:04:41 NyaVM-VPS-updwugzl squid[21868]: squid: No running copy

个人感觉：

• 很多时候起一个服务是起不来的
• 不管我是否起来了，感觉它会在服务启动的一瞬间就去执行 stop 的相关任务

有没有人有这方面的经验？