Good language learners become teachers paradox
The good language learners become teachers paradox (/gʊd læŋgwɪdʒ lɜ:nəz bəkʌm ti:ʧəz pærədɒks/) arises from the observation that language teachers are often people who are (or were) good language learners, but that good language learners are rare.
Most L2 learners do not achieve high levels of linguistic competence, and the evidence suggests that those who do may have done because of unusual degrees of aptitude and/or motivation. The paradox is that these good learners then go onto study language at university and thereafter become teachers, who now at least find it difficult to appreciate the difficulties students have with language because they didn't suffer similar problems themselves. Worst is the attitude "well grammar-translation worked for me when I was at school thirty years ago, so that's exactly what I'll use".