Many of us take things for granted, but have you ever thought about where that saying came from, or did you just take it for granted? There are many definitions, like to seize with authority, or to take with force or skill and more. The one I like is, 'to expect someone or something to always be available to you'. Have not been able to find a definitive source, but it seems that in the old days, rulers used to grant use of land, use of title, etc., and sometimes people would just assume possession of these things, even though they had not been officially granted to them. Hence, take it as if it had been granted to them.
Usually this has a negative connotation, such as being taken for granted. However, sometimes in relationships it can be good to be 'taken for granted'. It means someone has come to rely on you to always be there or always react in the same way. I take for granted that every Friday will be a Happy Friday.