1. I am trying my best to help you because I love Lou.
2. Valerie Sun yelled because she won the lottery.
3. Ted and Rosie went to bed because they were drowsy.
Do we need a comma to separate the main clause and the because-clause?
1.Because I love Lou, I am trying my best to help you .
2. Because she won the lottery, Valerie Sun yelled .
3. Because they were drowsy, Ted and Rosie went to bed .
When because follows a negated verb phrase, it must be preceded by a comma when the because clause explains why the event did not occur. They didn’t want her on the committee, because she was so outspoken means roughly “Her outspokenness was their reason for not wanting her on the committee.” When there is no comma, the because clause is included in what is being negated. Thus They didn’t want her on the committee because she was so outspoken implies that they may in fact have wanted her on the committee but for some reason other than her outspokenness.
http://www.bartleby.com/64/C001/015.html