Just because someone is strong doesn't mean that they don't need help. Just because they cope with whatever comes their way doesn't mean you can keep dumping on them and not offering support.
I've noticed with a few of my friends that I have to ask how they are twice. The first time I ask they say they're fine. The second time I ask, I get the truth.
If you've got a friend who's strong, next time you seem them ask how they are and really listen. Don't ask then start talking about yourself, ask and wait. Give them time to talk. You might be surprised what you hear.