Significantly, most people are not symmetrically bilingual. Many have learned one language at home from parents, and another later in life, usually at school. So bilinguals usually have different strengths and weaknesses in their different languages—and they are not always best in their first language. For example, when tested in a foreign language, people are less likely to fall into a cognitive trap (answering a test question with an obvious-seeming but wrong answer) than when tested in their native language. In part this is because working in a second language slows down the thinking. No wonder people feel different when speaking them. And no wonder they feel looser, more spontaneous, perhaps more assertive or funnier or blunter, in the language they were reared in from childhood.
What of “crib” bilinguals, raised in two languages?
Even they do not usually have perfectly symmetrical competence in their two languages. But even for a speaker whose two languages are very nearly the same in ability, there is another big reason that person will feel different in the two languages. This is because there is an important distinction between bilingualism and biculturalism.
Many bilinguals are not bicultural. But some are. And of those bicultural bilinguals, we should be little surprised that they feel different in their two languages. Experiments in psychology have shown the power of “priming”—small unnoticed factors that can affect behavior in big ways. Asking people to tell a happy story, for example, will put them in a better mood. The choice between two languages is a huge prime. Speaking Spanish rather than English, for a bilingual and bicultural Puerto Rican in New York, might conjure feelings of family and home. Switching to English might prime the same person to think of school and work.