1. You can't trust a random partner to do their share.
Whether you get to choose your own group or not, there is always that one person you are meeting for the first time. Do they get their work done on time normally? You will never know until you get started on the project.
2. If you want the project done right, you have to do the whole project yourself.
There comes a time in the project when you just decide to do it all on your own. You'd rather just take on the whole load yourself than let anyone else even attempt to try.
3. Some people seek out the hard workers of the class to get a free ride to a good grade.
We all can agree that these individuals are the absolute worst. They are banking on you to get a good grade, but will never put any work into it.
4. Most of your grade is falling on someone else's shoulders.
If you do end up getting people in the group that will help, you are still putting your grade on the project in someone else's hands. Refer to #2 when you don't trust anyone with that kind of responsibility.
5. Not everyone will be happy with the topic that is chosen and those individuals will continue to bring that up every chance they get.
When deciding on a topic for the project, or just which direction to take the project, not everyone will agree. Then, when the project hits a wall, they bring it up that everyone should have listened to them. If everyone would just have listened to their idea there would be no wall.... As if that were ever true, but you will still never hear the end of it.
6. Finding times that everyone can meet is impossible.
Some group mates make their schedule to work around the meeting times. These individuals are gold. Yet, there are others that will always plan things on top of the meeting times just so they have a reason not to show up. Even if that reason is complete BS and everyone knows it.
7. Some individuals believe in last minute fate.
The "oh we still have time" individuals. No, there is no more time and it would be so much easier if the project gets done ahead of time when there were the set-out meetings to get them done. When the majority of the group is this way, it's just easiest to plan on getting the whole project done the night before.
But once it is all over with you know that the group evaluations are not going to be so fun for some individuals and you hope that you never have to see them again.