Here it is in no particular order. Just how it comes to mind. SPOILER WARNING!!
1: His father is deputy and is too busy to pay him much attention.
2: His father dies when he's still a kit.
3: He has to go on a journey right when he was about to become a warrior.
4: On that journey he had no one. (For a while, anyways.)
5: On the way back home from the journey, Feathertail dies saving him.
6: His warrior ceremony is postponed forever basically.
7: Leafpool hates him for a while. (Even though it never says if he has knowledge of this.)
8: Leafpool doesn't show up to the meeting they plan so he has to wait all night in da cold.
9: He has a dream about Feathertail's death.
10: After said dream Leafpool dumps him.
11: He has to see Leafpool's clan get torn apart by badgers.
12: He has to see Leafpool grieve over Cinderpelt and do nothing about it.
13: The Windclan cats are jerks toward him after he rejoins the clan.
14: Webfoot is rude to him at a gathering in front of everyone.
15: He feels like he has to mate Nightcloud to prove his loyalty. (You may not see that as sad but imagine if you had to marry someone just to get accepted by your family?)
16: One of his kits is born dead and one lives for only a few moments.
17: The only surviving kit hates him.
18: He suddenly discovers that Leafpool had his kits.
19: Leafpool's kits hate him and Leafpool.
20: Hollyleaf announces that she and her brothers are half-clan at a gathering.
21: He realizes that Leafpool never told him about his kits even though he had a right to know. (I know, I should've put this earlier.
22: He witnesses Lionblaze and Breezepelt fighting.
23: Leafpool yells at him during said battle in front of his patrol.
24: His mother gets her throat clawed out by some Dark Forest idiot.
25: His mentor becomes evil. (Should've put this earlier, too.)
26: Breezepelt attempts to kill Lionblaze.
27: Breezepelt is happy that Hollyleaf dies.
28: Onestar is mean to him about grieving over Ashfoot's death.
29: His kits all hate each other and Leafpool.
30: Onestar exiles him from Windclan.
31: He has a dream that makes him think he's going to the Dark Forest.
32: He gets in trouble for taking Heathertail, Gorsetail, Hootpaw (I think it was Hootpaw but I could be wrong.), and Breezepelt away to find Nightcloud.
33: Nightcloud disappears. (Should've put that earlier as well.)
34: Onestar won't listen to him and ask for help.
35: Leafpool dies. (Sorry for the huge time-jump, I haven't read the fifth arc yet.)
36: He's named as a codebreaker.
37: He gets exiled for supposedly being a codebreaker.
38: Lionblaze is a jerk when they're both in the Rebels.
39: He attempts to commit suicide when Feathertail dies. (Come on, your life must be sad if you do this.)
So yeah. I probably missed some because I didn't think of it/haven't read all of the books.
