How can I make my interests last longer?

I am not always a very passionate person. I have many passing interests. For example, there are times when I strongly want to learn French. This interest might be serious for a short span of time, but then I don’t care if I learn it anymore. I don’t like this because I think knowing and understanding French where I live is pretty important, but my mind seems to be too lazy as I get bored and give up too easily. How can I make sure I stay interested in learning French without getting bored or giving up?