Following are a few examples:

Supposing White, after (1) P-e4, P-e5, plays (2) P-d4. Then Black
is immediately threatened with the loss of his center-Pawn, which
he cannot defend with P-d6 as after (3) Pxe5, Pxe5 White would
exchange Queens and thereby deprive Black of the possibility of
bringing his Rooks quickly into cooperation through castling.

(2) ..., Kt-c6 is out of question too; not on account of (3) P-
d5, which merely helps Black in getting his Knight over to the
King's wing via e7 and allows him to maintain his center-Pawn,
but on account of (3) Pxe5, Ktxe5; (4) Pf4, which secures the
control of the center for White and does not leave Black a chance
to advance his Queen's Pawn to d5 with the view of depriving
White of his center-Pawn too.

For these reasons Black has no choice but has to play (2) ...,
Pxd4. True enough, Black gives up his center-Pawn with this
exchange, but White cannot prevent P-d5 in the long run, so that
he also loses his center-Pawn and has no advantage over Black in
that respect. The following variations are typical for the line
of play in cases of this kind. White tries to hold back Black's
Queen's Pawn as long as he can, but finally Black enforces the
advance.

I. (3) Qxd4 Kt-c6
(4) Q-e3

Now P-d5 is not possible because of Pxd5 check.