2-) If your dps is low or you are just simply need more training please dont try to join more experienced players lfgs. You should first go "training runs" than you should find people who has same experience like you to train more for reach more gucci gameplay. You dont need to get mad at experienced people who dont accept you in lfgs.
That is not how it should be though.
First of all, most low dps players don't realize their dps is low, secondly just training does not fix a wrong rotation or understanding of the class.
You need to play with better people to learn from their positioning and in some cases rotations, not play with equally bad players.
If a group doesn't specify they want people with x amount of clears then new players should be free - even incentivised - to join such parties in order to get better.
Getting mad for being declined is bad, yes, but I don't agree with the rest.