But I just cant help falling in love with you...
To a point you can control who you fall in love with. For example, if you know that you fancy someone on first meeting them, but know that it is a relationship you should not pursue, you can make sure that you don't contact them and don't do anything that is likely to encourage any further interaction between you. However, sometimes it just happens. Maybe you have to spend time with them because of school classes, homework, or maybe you work with them. Love usually grows out of the initial admiration, but there are a lot of boxes that need to be ticked to fall fully in love with someone. Just because you admire somebody, maybe you admire how he/she looks, maybe his/her intelligence, maybe how the person cooks, and things like that, don't all simply add up to falling head over heels for someone.
But there are things to be considered depending on the situation. If you're falling in love with a person that has a relationship, no matter how long they've been together, if they're engaged, married or just living together you should start looking away and try to spend less time with this person, sometimes is hard if you have to work with him/her, specially if you are in constant communication with this person, but it is possible. The thing is, that no matter what, even if he/she says the relationship he/she is in now is going down and their getting divorced, you should step back. Never do to others what you wouldn't like others do to you.
Another good example of this is that some people who did not know they were homosexual, can suddenly realise that they are in love with their best friend. Often, we don't always choose the easy option, and in some strange way, love that is won and forged through arduous tasks and difficult times, seems to form a really strong bond between partners. The Shakespeare quote from a Midsummer Night's Dream phrase "The course of true love never did run smooth" explains pretty well, that falling in love is rarely simple or easy, or without its tribulations.