Dale Carnegie puts up in his book ‘’, never use ‘but’ after a positive statement. Replace it with ‘and’.
For example : You are good at studies but you must work harder. Introduction of ‘but’ negates all the information before it. This looks phony and insincere.
Better example : You are good at studies and you must work harder. Here conjunction ‘and’ does not negate previous appreciation and rather enhances the total positive effect.
For negative statements ‘but’ can be used positively.
Example : Your English is bad but you are improving fast.
Hence use ‘and’ to connect similar statements (positive) and ‘but’ to connect contradictory statements.
Famous Use : I love you but as a friend!!