Why does practice (but not practise) make perfect? The answer depends on the English dialect you intend to write the word in. If you write in United States English, you needn’t worry about the word “practise” at all—US Americans always spell the word with a -c-, and if you use an American spell checker and type in “practise”, you will likely see a squiggly red line underneath it.
If you write in British, Canadian, Australian, New Zealand, or Irish English, you must distinguish between “practice” and “practise”:
practise = a verb, as in “he practises law”
Think about what the word expresses; is it an action (a verb), or is it a thing (a noun)? If it is a thing someone has, wants, needs or that does something, it is spelled with a -c-:
When somebody or something practises, i.e. it is something they do, it is spelled with an -s-:
It is worth noting that the spelling only depends on the part of speech (whether it is a noun or a verb), not on the intended meaning. Hence, in non-US English, you can “practise Christianity” but not “practice Christianity”, as well as “go to a doctor’s practice” but not “doctor’s practise”.