Work/feeling left out

Have you ever felt left out at work? I have for awhile now. I try my best to communicate with all of my coworkers and engage in relatable conversations. We are a small office, only 8 of us, and sometimes we will have outings. It has happened twice now where I have not been invited to those big outings and instead had to spend the day in the office. I have been trying so hard to look at the positive and be thankful for the position I have and the opportunity. Am I overthinking these situations? I am also worried it is starting to make me depressed.