Kind of like having a beginner swimmer jump into the deep end.
Many math departments now recommend or require an introduction to proof writing course, or a sophomore level course including proof writing (e.g. discrete math or a theory focused version of other typical courses) before taking abstract algebra or real analysis.
Since you referenced Harvard Math 55, note that Harvard says that it is for students who already have experience with proof writing. Students who want to learn proof writing are advised to take Math 23 or 25, not 55.