Your centre stone is in atari: three white stones have taken three of its four liberties, and if it stays still it comes off the board. In atari you have exactly two choices — run or die. Extend to the one open side, where the board is wide, and the new stone contributes its own liberties: the pair now breathes three times. The habit to build is running toward open space, never along a wall or into a crowd, because liberties multiply fastest in the wide centre.