assumes the brunt of responsibility in protecting his team and carrying out their mission.

While Sasuke is at times frustrated with his teammates' foolish incompetency, he's never particularly tears into them for their limitations either but rather accepts them and adapts and deals with his team's mistakes or shortcomings. When on the field, he doesn't blame Sakura for being incapable of protecting herself (even though she is) and he doesn't give Naruto an especially hard time for his dumb mistakes (even though they are quite dumb), but rather deals with their limitations and accommodates them to the best of his ability. 

Naruto and especially Sakura praise Sasuke's abilities too, so team praise and encouragement is not exclusive to Sasuke, as the three genin all praise each other at different intervals. What is different, though, is that Sasuke actually shows consideration for his team's limitations and accommodates them, going so far as to allow his teammates breaks even in the middle of a battle. You won't often see Naruto and Sakura offering Sasuke a break in the middle of a fight or telling Sasuke he can sit on the sidelines because they have it handled, because they don't, and therein lies the whole leadership aspect (except for in the fight with Gaara when Naruto promises to handle things so Sasuke doesn't have to, but that was a point of major growth for Naruto and a year into the team's formation, and also bravery that Naruto wouldn't have found on his own without Sasuke first promising to sacrifice himself so that Naruto and Sakura could escape). Naruto and Sakura are reliant on Sasuke as their leader, but Sasuke can't yet be as reliant on them because the two kids simply aren't yet strong, skilled, or mature enough yet.

Even small details emphasize the leadership aspect, like the scene before Hiruzen's funeral where the genin trio gathers for the ceremony. Naruto and Sakura wait for Sasuke and don't make a move until Sasuke arrives. Sasuke strides past them, then stops, silently beckoning them to follow, and it's only at this nonverbal "order" that Naruto and Sakura run after Sasuke, following their leader to the funeral. It may seem like a small and subtle gesture, but even tiny moments like this show that Sasuke is the team's unspoken leader, and Naruto and Sakura are his loyal followers, dependent on Sasuke's strength and support in the team.

Thhis role of leadership puts a lot of pressure on Sasuke. Not only is Sasuke obligated to look out for himself, he's expected to take care of Naruto and Sakura as well. Considering how much trouble the three of them manage to attract, this is an exhausting task, too heavy a burden for a 13-year-old. In the end, the weight of such responsibility proves too heavy for Sasuke and something he doesn't yet have the maturity for, and the pressure builds up over time, and eventually becoming another indirect component in the reason for his desertion.

Naruto and Sakura largely take Sasuke's protection for granted and even overlook it, but maintaining a regular awareness of his team and keeping his comrades safe all the time isn't easy, and it often results in Sasuke having to take a lot of hits, throwing himself fearlessly into danger and getting seriously injured on behalf of his teammates. It's being a leader, and it's stressful and exhausting for young Sasuke.

While he doesn't mind protecting his team, at such a young age, it's impossible to continue with that level of stressful responsibility without sufficient emotional support. Naruto and Sakura were maturing rapidly and growing in maturity in leaps and bounds, but they were still too naïve and immature to yet be substantial emotional support that Sasuke needed for such a draining task, particularly when the criminals they face started becoming S-class criminals.