Because it's important to recognize and deal with them sometimes.
  1. The awful nagging sensation that it isn't really "all right" no matter how many times your (spouse, friend, relative) explains to you that they forgive you for whatever stupid mistake you committed.
  2. The sinking feeling after opening up about your honest feelings for someone and being rejected on some shallow basis.
  3. Not being truly heard. No matter how specific and carefully worded the sentence is your true meaning flies past closed ears. Especially hard when you are misunderstood in a negative way.
  4. Feeling disconnected from others. As humans we all crave some kind of interpersonal connection. Sure we can live without it for some time but it's just not fun at all. The worst being surrounded by fake people and playing along with the niceties.
  5. Hurting someone's feelings unintentionally. Saying something with the best intentions and then seeing it taken as an insult by the second party. No matter how hard you explain yourself they just can't see that you meant so much better than you sounded.
  6. Giving up. Trying so hard only to throw in the towel at the last minute. Seeing months later where you could have been had you stuck around long enough to try.
  7. That feeling in your stomach when you have so much to do that you want to do well but have also procrastinated excessively so you feel hopeless and overwhelmed.
    Suggested by @shreyasaurus