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In Deathly Hallows we learn Snape was in love with Lily, so he risked his life becoming a double agent in order to protect Harry.
But in Order of the Phoenix, instead of strengthening Harry's protection against legilimency, he weakens it, granting Voldemort access to Harry's mind.
Then, why did he risk Harry's life, if he was risking his own life to protect him?
I don’t think he’s intentionally weakening Harry’s mind; it’s a by-product of the nature of the lessons. I think this is the passage you’re referring to:
“Hermione told me to come and check on you,” said Ron in a low voice, helping Harry to his feet. “She says your defenses will be low at the moment, after Snape’s been fiddling around with your mind…. Still, I suppose it’ll help in the long run, won’t it?”
He looked doubtfully at Harry as he helped him toward bed. Harry nodded without any conviction and slumped back on his pillows, aching all over from having fallen to the floor so often that evening, his scar still prickling painfully. He could not help feeling that his first foray into Occlumency had weakened his mind’s resistance rather than strengthening it, and he wondered, with a feeling of great trepidation, what had happened to make Lord Voldemort the happiest he had been in fourteen years.
— Order of the Phoenix, chapter 24 (Occlumency)
Their lessons involved Snape repeatedly attempting to break into his mind. That drains him of energy and naturally leaves him more vulnerable.
The intention was never to weaken Harry, but occurred because of Harry’s ineptitude and the way that Snape was teaching him.
This quote explains it:
“Snape made it worse, my scar always hurt worse after lessons with him —” Harry remembered Ron’s thoughts on the subject and plunged on. “How do you know he wasn ’t trying to soften me up for Voldemort, make it easier for him to get inside my —”
“I trust Severus Snape,” said Dumbledore simply. “But I forgot — another old man’s mistake — that some wounds run too deep for the healing. I thought Professor Snape could overcome his feelings about your father — I was wrong.”
— Order of the Phoenix, chapter 37 (The lost prophecy), emphasis mine
Then:
I find it somewhat nonsense, but seems the canon answer.
Actually it's very like going to the gym.
You going in as powerful as you can, you doing weight lifting and other exercises and when you leaving the gym, your legs and arms are like jelly at the beginning. You are weaker than you went in. Your muscles are hurt, you don't have too much energy at all. But with more practice you getting stronger and stronger.
It can be less dramatic if you doing it with "thinking", but can be very painful if you "overdoing" it.
Snape wasn't really kind, that's true. He wanted to train Harry on "full speed" from the first second... it's like trying to lift 100 KG on your first day in the gym. He is not a good coach because of his feelings about Harry (he hates him yet is protecting him) - he simply does not have the patience required for this training.
The last thing he wants to do is to hurt Harry, though - there are several occurrences/cases in the series where Snape protects him.
He is just not the right person for the job - yet he is the best person for the job as he proves he can hide things from Voldemort.
I'd say the temporary weakness is the nature of the training but the small success is because of Harry himself (he resists and wants to use the connection to look into Lord Voldemort's mind) and Snape's lack of patience.
Harry refused to cooperate with Snape during his instructions, so instead of getting stronger every week he got worse because of the lack of trying and constant invasion by Snape it left him weak. It was Primarily Harry's fault, though, because he would not work with Snape. But Snape was being frustrating anyway.
As extraordinary wizard as he is, Snape is unfortunately obviously a poor teacher.
The best proof is the Harry's week performance on Potions, where he (ironically, using Snape's instructions) perform extraordinary on Slughorn's lessons.
You remember first Potions lesson when Snape ask Harry questions that would require reading book in advance? Snape has unrealistic expectation, based on his own untypical live experience, which most pupils (with exceptions like Hermione) can't fulfill. Instead of steadily introduce Harry to the hard art of Occlumancy, he overloads him, exhausting him and making things worse.
It's like forcing the beginner to run the marathon on the first lesson. You're more likely to cause serious damage than to train someone. Being extraordinary sportsman doesn't make you good trainer, the same obviously applies to wizardy.
Snape was more interested in enjoying Harry's pain than teaching him. He could have given him reading material or explained how he and others closed their mind. Maybe he could have taught him meditation to clear his mind or attacked his mind with less power until he began to get the hang of it. Snape never changed tactics when it became clear his teaching style wasn't working. Harry should have practiced but he didn't trust Snape and thought he was trying to make things worse. Considering the way Snape treats him I don't blame him.
