They say a change is as good as a holiday, sometimes if a routine gets to be a mental drag a new routine can get the motivation levels back up, especially at times in your life when you have other important stuff going on. Sometimes the old routine just isn't suitable anymore.

I've been training for over thirty years and my routine hasn't stayed fixed for much longer than 12-18 months max. It can be mentally challenging to change a program that you feel has worked well ( a bit like breaking up with a long standing girlfriend) but you get over it quite fast when you realise the new one is good in a different way.