Explanation of my game. Can someone explain the "why" to me? Ok, here's a game that I won. I was playing vs someone on chess.com, and playing as black. My PGN is as follows (with comments from the analysis engine):

[fen ""]
1. d4 e5 2. Nf3 {MISTAKE (-0.26) The best move was 2. dxe5}
(2. dxe5)
2... Nc6 {MISTAKE (+1.33) The best move was 2... e4}
(2... e4 3. Nfd2 Nf6 4. c4 c6 5. e3 d5 6. Be2 Be6 7. Nc3)
3. Nc3
3... Bb4 {INACCURACY (+0.98) A better move was 3... exd4}
(3... exd4 4. Nxd4 Nf6 5. g3 Bb4 6. Bg2 O-O 7. O-O d5 8. Nxc6)
4. dxe5 Nxe5 {MISTAKE (+3.60) The best move was 4... d5}
(4... d5)
5. Nxe5 Bd6 6. Nd3 Nf6 7. e3 b6 8. Qf3 Rb8 9. Qe4+ {BLUNDER (-4.53) From winning to losing. The best move was 9. Nb5}
(9. Nb5)
9... Nxe4 10. Nxe4 Qe7 11. Nxd6+ cxd6 12. f3 Qh4+ 13. Nf2 Qb4+ 14. c3 Qc5 15. Nd3 Qd5 16. e4 Qc4 {INACCURACY (-1.22) A better move was 16... Qe6}
(16... Qe6 17. Nf4)
17. Nf2 {INACCURACY (-2.60) A better move was 17. Nb4}
(17. Nb4 Qe6 18. Bf4 f5 19. Nd5 Qg6 20. exf5 Qxf5 21. O-O-O Bb7)
17... Qc6 {INACCURACY (-1.56) A better move was 17... Qe6}
(17... Qe6)
18. Nd3 {INACCURACY (-2.60) A better move was 18. Bf4}
(18. Bf4 b5 19. Rd1 Rb6 20. Bxd6 Qb7 21. Rd5 b4 22. Bd3 bxc3)
18... O-O 19. Nf4 {INACCURACY (-3.83) A better move was 19. Nb4}
(19. Nb4)
19... Re8 20. Bc4 {MISTAKE (-7.39) The best move was 20. Kf2}
(20. Kf2 f5)
20... d5 {BLUNDER (-2.20) The best move was 20... Qxc4}
(20... Qxc4)
21. Bxd5 Qc5 22. Bb3 d5 {MISTAKE (-0.87) The best move was 22... Bb7}
(22... Bb7 23. Nd3)
23. Bxd5 {MISTAKE (-3.69) The best move was 23. Nxd5}
(23. Nxd5 Be6 24. Be3 Qc6 25. O-O f5 26. Rad1 fxe4 27. fxe4 Rb7)
23... Bb7 24. Bd2 {INACCURACY (-6.92) A better move was 24. Bxb7}
(24. Bxb7)
24... Bxd5 25. Nxd5 Rbd8 {BLUNDER (-1.94) The best move was 25... Qxd5}
(25... Qxd5 26. Rd1 Qxa2 27. Bf4 Rbc8 28. Bc1 f5 29. O-O fxe4 30. f4)
26. O-O-O Rxd5 {INACCURACY (-1.14) A better move was 26... Qc4}
(26... Qc4 27. a3 f5 28. Ne3 Qa2 29. Nxf5 Rxd2 30. Kxd2 Qxb2+ 31. Ke3)
27. exd5 Qxd5 28. Be3 {INACCURACY (-2.55) A better move was 28. Rhe1}
(28. Rhe1)
28... Qxa2 29. Bf2 {MISTAKE (-6.58) The best move was 29. Rhe1}
(29. Rhe1 f6)
29... Re2 {BLUNDER (♔ MATE IN 2) From winning to losing. The best move was 29... Qa1+}
(29... Qa1+ 30. Kc2 Re2+ 31. Kd3 Qxb2 32. Bg3 Rxg2 33. Rhe1 Qc2+ 34. Kd4)
30. Rhe1 {BLUNDER (♚ MATE IN 1) From winning to losing. The best move was 30. Rd8+}
(30. Rd8+ Re8 31. Rxe8#)
30... Qa1#


And here are the "stats" from the game:

Strength    White   Black
Excellent   18  15
Good    1   5
Inaccuracy (?!) 5   4
Mistake (?) 4   3
Blunder (??)    2   3
Forced  0   0
Best Move   32.1%   26.7%
Avg. Diff   1.39    1.21


The analysis engine tells me what the mistake was, and what the best move was, but not really "why". I'm more interested in the mistakes and blunders at this point.

I'm really just trying to learn right now...any thoughts or advice are really appreciated.
This is a game between beginners. Nothing wrong with that, but there is not much point at this level giving a very deep explanation as to the "why". Both players should pay more attention to 


not blunder pieces in one move
not miss mate in one move


Concretely: 


From move 1 to move 4 a pawn was hanging on e5. White could/should have taken it earlier and black should not have allowed that (not move it there in the first place and later prevent it from being lost by either moving it away playing e4 or by capturing exd4.

...Nxe5 is a big mistake as it just drops a piece. Always check and double check if your pieces are attacked.


Qe4 just drops the queen. As above with every move you make, check whether your opponent can capture something (particularly if it is your strongest piece...)


Bc4 is another big mistake as it drops the bishop. Black should have responded with 20...Qxc4 winning a full piece for nothing.

22....d5 is a mistake because it drops a pawn.
24 Bd2 is a big mistake as it drops another piece. Admittedly this is a bit more tricky to see. Basically the bishop on d5 is protected only once by the knight on f4, because the pawn on e4 is pinned to the king, so you cannot recapture e4xd5. Black could have played 24...Bxd5 25 Nxd5 Qxd5 winning a piece.

Bf2 is a mistake because after 29...Qa1+ black will eventually win the pawn on b2 and the white king will be without protection in the middle of the board. For instance 29...Qa1+ 30. Kc2 Re2+ 31. Kd3 Qxb2 etc. A better move would have been 29. Rhe1 which prevents the black rook from entering with check via e2. Admittedly at your current level, this is not a big oversight unlike the other mistakes and I would not yet worry about it.

29....Re2 is a terrible mistake because it would lose to back-rank mate after Rd8+. Back rank mate is an extremely common tactics and you have to be aware of it at all times. You need to have the typical position (=three pawns in front of castled king and opponent queen or rook) ingrained subconsciously in your chess brain.




So in summary: 

Pay more attention to hanging pieces (both yours and your opponent's and learn some basic tactics.