Why did I lose this game? I can't believe it!!

Well. Its the problem of games without mistakes.

The strategie is more complex.

All chess games have mistakes.  It is an error to think that because you didn't blunder a piece the game was without error.  With that said:

Move 4 and 5 are mistakes by both parties.  Breaks a opening law "do not move the same piece twice in the opening.  Obviously this law can be bent if the overall goal improves position and development.

I looked over the entire game and it looks like white just had a better position the entire game and black was happy to continue trading down with no thought. 

Notice the the 3 pawn chains of Black to the 2 pawn chains of white.  It is clear that white will be better if black pushes for trade of rooks from move 20.  Black should be playing for pawn play on the kingside in this position.

Position Board on move 20 seems to be the key position in the game.  I see the game a materially even and the position drawn (through human eyes, computer might give slight advantages to one side or the other, but I would doubt any major advantage.)

The looks like a draw to me from this position and I would have offered it after the rook trades.

Move 22 you could have won a pawn by taking (Rxb2)... and the game would be completly even draw.

Hope this helps...didn't really take much time to analyse.

Well move 4.-...Nd4 its the spanish variation of 4 knight opening.

I also suggest that you study the four knights opening!!I think  you dropped a pawn when you play Nd4 on move four,all white has to play is Nxe5 on move five!When white plays Bc4 you should have played d6, instead of on move seven Bd6 for black is good,you want to keep that attack up and keep making white fight on,I suggest also that you study endgames but more importantly study tactics,Rxb2 would work for black as well,remember your rook needs to be behind passed pawns and behind your opponents pawns!!also instead of ur pawn take on d5 ,you could use ur knight it would be better instead of leave the d5 pawn by its self!!

instead of 21...Rc2, first 21...f6 and then you play Rc2 - should draw easily I think (you should be able to snatch b2 at some stage)

4... Nd4 followed by 5... c6 hangs a pawn

The exchange that follows 6... d5 leaves you with an ugly isolated queen pawn

9... Nxf3+ doesn't seem like the best move, nor does 10... Qe7+ as it allows for your opponent to continue to trade down to the point where his pawn structure and material (one pawn) advantage will be invaluable.

16... Bxg3 looks nice, given that it doubles your opponents pawns, upsets his king cover and opens up e2 for your rook to enter your opponents position, but the drawback is that you've just exchanged down to a knight vs. bishop in a very open position.  Your opponent never made use of his resulting superior minor peice becuase you traded them off, but it's something to watch for.

Getting your rook deep into the enemy position was good, but 28... R8xe3 leaves your bank rank weak -- you might have been better to recapture with the rook on the 2nd rank instead.  The weak back rank helped your opponent to force yet another exchange with 21. Rfe1.

24... Rc2 looks like a mistake -- it allows White to seize the open file with check and after this your rook is passively placed until it finally gets exchanged.

38... Rxb2+ exchanges the last peice off into a clearly superior endgame for White.  It wasn't a forced exchange, and taking back the open rank with Re1 looks better to me.

I think the bottom line is that once you dropped that pawn, and let your pawn structure become inferior to your opponents you needed to avoid letting your opponent exchange down to a winning endgame, which is exactly what he managed to do.

4...Nd4 is Rubinstein's line perfectly good pawn sac, if know what your doing. 5...c6 and 6...d5 played by Svidler and Hebden amongst others...  Maybe get more compensation for pawn with 7...Bd6 instead of 7...pxd5.

   After 22Re1 looks like you can get away with 22...Rxb2 23Rxe3 Rb1ch, like somebody said earlier.

To me it seemed like black was more than happy to trade down....so what was the use of sacrificing the pawn in the first place?

u lost becuz your king was not active in endgame while white's king battled alongside his rook and pawn.

learn patience.

It seems to me that you kept your rook unnecessarily occupied on the opponents side in the end when it was more required on your side guarding your heading pawns (more on pawns on a and b files).Rook sacrifice was a mistake here according to me as your opponent had better & strong pawn chain on a,b & c files.

The root of the problem to me seems to be that you allowed white to win a pawn and leave yourself with an IQP - this in itself isnt decisive but if you are playing with an IQP you need to strive for active minor pieces - white's strategy against an IQP is to trade off minor pieces to weaken the defence of the pawn - you assisted white to do this in trading down - then tackling the weakened queenside pawns was a matter of technique for white ..to create an outside passed pawn - I think maybe watching the video by Yasser Seirawan (Pro Chess) might help - it explains this area quite well ... if you can ignore his Saturday Night Fever suit!!

TheGrobe above seems to have covered this quite well

Thanks for all.