Listening is a musician's most valuable skill. But how you listen is just as important as how well you listen.
Critical listening means listening for flaws. With highly trained ears, you can hear when a note is slightly flat or sharp, when a player is slightly ahead of the beat or behind it, and when a guitar tone is too dull or too intense. An untrained ear can sort of hear that something is a little off, but your trained musical ear can hear exactly what's wrong, and hopefully you can take steps to fix it.
Critical listening is essential, but if you do it all the time, you will just make yourself unhappy, and probably everyone around you too! It's just as important to be able to do casual listening: experiencing the song the way a typical audience member will listen to it. That means being uncritical, detached, unfocused, and not even paying full attention.
Critical listening will help you get the details right. Casual listening will help you have a successful song. Make sure you know the difference and that you develop both of these skills.