We can feel worried about our friends for many reasons. Friends, just like us, can feel lots of different emotions.
They may usually appear happy and chatty and then become quiet, not talk to us as much as before or seem to be very sad a lot of the time. They might not want to go out and hang out as usual, or, when they do, they might not seem like their usual selves.
Some people find it easy to tell other people what is on their mind. Others can find this really difficult. It is worth remembering that Talking Makes Us Stronger.