user13862: I recently came across 1.d4 Nc6 while playing a game and I was immediatly tempted to play 2.d5, and so I did it, then my opponent continued the game with Ne5. 
After the game I went to an online database to check some reference on this opening, I saw it is called the Mikenas Defense, and to my surprise 2.Nf3 and 2.e4 are by far the most common responses to it, while 2.d5 occurs only in 5% of the games. I'm a ~1400 player and I'm still learning the basics of openings theory, I can understand that both of these moves help white to develop its pieces, but can't they be naturally developed after 2..Ne5(which seems to be the most played move after 2.d5) with something like 3.e4? 
[fen ""]

1. d4 Nc6
2. d5 Ne5
3. e4


user9627: D5 is fine to play, but it is commital and a bit lacking in mobility. I probably wouldn't play it. 
Nf3 does a good job preventing a quick e5, and e4 claims the center nicely. Neither are too commital and leave room for all kinds of options.

user12449: Just the opinion of a 2000 player.

[fen ""]

1. d4 Nc6 2. d5 Ne5 3. f4 Ng6 4. e4 e6

The idea of this opening, and others like the Alekhine and the Two Knights Tango, is to lure the white pawns forward to make them easier to attack.  Black gets a very cramped game, but it is playable.

[fen ""]

1. d4 Nc6 2. Nf3 d5 3. c4

The main purpose of Nf3 is to prevent black's knight from using that square.  After normal moves, black will find that blocking the c-pawn advance is considered bad.
One of the general objectives in openings is to control the center.  White can attack black's center with the c-pawn, yet black can't support this with his own pawn.  This leads to a small increase in white's advantage.

13862: <p>I recently came across <strong>1.d4 Nc6</strong> while playing a game and I was immediatly tempted to play <strong>2.d5</strong>, and so I did it, then my opponent continued the game with <strong>Ne5</strong>. </p>

<p>After the game I went to an online database to check some reference on this opening, I saw it is called the Mikenas Defense, and to my surprise <strong>2.Nf3</strong> and <strong>2.e4</strong> are by far the most common responses to it, while <strong>2.d5</strong> occurs only in 5% of the games. I'm a ~1400 player and I'm still learning the basics of openings theory, I can understand that both of these moves help white to develop its pieces, but can't they be naturally developed after <strong>2..Ne5</strong>(which seems to be the most played move after <strong>2.d5</strong>) with something like <strong>3.e4</strong>? </p>

<pre><code>[fen ""]

1. d4 Nc6
2. d5 Ne5
3. e4
</code></pre>
 9627: <p>D5 is fine to play, but it is commital and a bit lacking in mobility. I probably wouldn't play it. </p>

<p>Nf3 does a good job preventing a quick e5, and e4 claims the center nicely. Neither are too commital and leave room for all kinds of options.</p>
 12449: <p>Just the opinion of a 2000 player.</p>

<pre><code>
[fen ""]

1. d4 Nc6 2. d5 Ne5 3. f4 Ng6 4. e4 e6
</code></pre>

<p>The idea of this opening, and others like the Alekhine and the Two Knights Tango, is to lure the white pawns forward to make them easier to attack.  Black gets a very cramped game, but it is playable.</p>

<pre><code>
[fen ""]

1. d4 Nc6 2. Nf3 d5 3. c4
</code></pre>

<p>The main purpose of Nf3 is to prevent black's knight from using that square.  After normal moves, black will find that blocking the c-pawn advance is considered bad.</p>

<p>One of the general objectives in openings is to control the center.  White can attack black's center with the c-pawn, yet black can't support this with his own pawn.  This leads to a small increase in white's advantage.</p>
