Many people think being assertive means walking all over people, but in fact it actually means that you confidently talk to others when you have something to say. This means that instead of waiting to let an issue build, you address it right away. Or that you confidently state your opinion, even though you know your friend disagrees. You do it with respect and don't argue, but you don't let someone else dominate the conversation either.
Being assertive means that when friends overstep their boundaries, you lovingly correct them. This helps keep arguments to a minimum and allows you to avoid those negative feelings that can build in a friendship over time.