The Manchester City winger scored the winner for England in their Nations League opener against Iceland, with his strike from the spot marking his 11th goal in 12 internationals, leading to the manager praising his "hunger".
The England manager said: "His mentality has been exceptional all week - [he's] desperate to be here, desperate to play."
Sterling has won 57 caps for his country at the age of just 25 and Southgate had some kind words about the star's determination to perform for England.
The 50-year-old coach said: "[His attitude] has a massive impact on everybody around him and really he has taken his game to another level.
"I can't speak highly enough of him really. It would have been very easy to have another few days on the beach, not report in, get himself right for his club at the beginning of the season."
Southgate also feels that Sterling's goalscoring ability has taken some of the pressure of captain Harry Kane to find the net for the Three Lions.
He said: "Harry [Kane] is a supreme goalscorer and is critical to the way we play. The nice thing is that we won't be leaning just on Harry now."
The boss also stressed to his players the importance of keeping 11 men on the pitch, with Kyle Walker dismissed against Iceland - the first time England have had a player sent off since he took charge.
He said: "It's the first time we've had a red card in my four years and it makes the game so difficult. It could have cost us the game.
"We got away with it but it's something all of the players ... it's a good way to learn, to still win the game, but to have to hammer those points home because it's critical."