1. 英语语法规定：如果用不定式to do sth做句子的表语，而句子的主语又含有行为动词do或其变化形式如does/did/done等时，则做表语的不定式的符号to必须省略。2. 即此两句本该是：1）All you can do is (to) just live.2) All I want to do is (to) travel.3. 再如：1）What I can do now is (to) wait.2) All that you need to do is (to) sit on your seat. 4. 此种语法现象一句话以言之，即为：前有do，后无to.
