피자 좋아해요. sounds like 'I like pizza'
but
피자가 좋아해요. would be 'Pizza likes something"
~가 / 이 are used to make a word 'subject', and ~를 is used to make a word 'object'
미리암이 피자를 좋아해요.(Myriam likes pizza)
Subject Object
누나가 피자를 좋아해요.(My sister likes pizza)
Subject Object