Castling a blunder?

According to the computer, castling was a blunder on my part. The computer may very well be correct but I don't see the blunder nor why d4 was the "best move". Would somebody please explain this?

1. e4 d5?! { (0.05 → 0.72) Inaccuracy. c5 was best. } (1... c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e6 6. Be2) 2. exd5 Qxd5 { B01 Scandinavian Defense: Mieses-Kotroc Variation } 3. Nf3?! { (0.66 → 0.13) Inaccuracy. Nc3 was best. } (3. Nc3 Qd6 4. d4 Nf6 5. Nf3 g6 6. Nb5 Qb6 7. Na3 Bg7) 3... Qe6+? { (0.13 → 1.57) Mistake. Bg4 was best. } (3... Bg4 4. Be2 Nc6 5. h3 Bxf3 6. Bxf3 Qe6+ 7. Qe2 Nd4 8. Qxe6) 4. Be2 Qg6?! { (1.49 → 2.51) Inaccuracy. Nf6 was best. } (4... Nf6 5. O-O g6 6. Re1 Qd6 7. d4 Bg7 8. c4 O-O 9. Nc3) 5. O-O?? { (2.51 → 0.74) Blunder. d4 was best. } (5. d4 Qd6 6. c4 e6 7. Nc3 Nf6 8. O-O a6 9. Qc2 c5) 5... Bh3 6. Nh4?! { (0.75 → 0.00) Inaccuracy. Ne1 was best. } (6. Ne1 Nf6 7. d4 Nc6 8. Bb5 Bd7 9. Nf3 O-O-O 10. Bd3 Qh5) 6... Qg5 7. d4?! { (0.34 → -0.44) Inaccuracy. Bf3 was best. } (7. Bf3 c6 8. d4 Qxh4 9. gxh3 e6 10. c4 Nd7 11. Nc3 Ngf6) 7... Qxh4 8. gxh3 Qxh3?! { (-0.30 → 0.50) Inaccuracy. Nc6 was best. } (8... Nc6 9. Bf3) 9. Bf3 Qc8 10. Nc3 c6 11. d5 e6?? { (0.81 → 5.37) Blunder. cxd5 was best. } (11... cxd5) 12. dxe6 Qxe6?! { (5.60 → 7.80) Inaccuracy. Nf6 was best. } (12... Nf6 13. Ne4) 13. Re1 { Black resigns. } 1-0

Can you post the game?

This is the game. TriboneSub was white pieces.

It is a mistake, because it pins White's g-pawn to their king, making it easy for Black to develop their light-squared bishop, abusing the pin and threatening mate in one and when it comes to d4, I guess it is just normal development and not exposing oneself

@TritoneSub
I have a great tool:
decodechess.com
It explains the reason behind moves in words and also tells why a move is bad.

Thanks for posting the game, kamilok04. (I wasn't sure how to do it so I just copy/pasted the moves).

If my understanding of the "Learn from my mistakes" is correct, then the computer would have me play e4 instead of castling. However, playing e4 instead of castling would have allowed black to play queen takes pawn on g2 forcing rook to f1 and destroying the king's side defences.

#6

If 5. Qxg2?! 6. Rg1 follows with a fantastic development for white :)

Ah ... I see ...
Thanks, davidm9. I was considering Rf1 but with the knight guarding, Rg1 is the move to make.

You castled into your opponents attack.