The 35-year-old singer - who is famed for his womanising ways and previously dated stars including Jessica Simpson, Jennifer Aniston and Cameron Diaz - is head-over-heels in love with the 'Firework' singer and loves that she has her own career and friends to focus on.
A source said: ''He loves that she's so independent and doesn't cling to him like a lovesick barnacle. Katy, of course, loves a bad boy but she's going to let a man stomp all over her.
''She's really quite sassy with John and he loves it. He needs someone to take him down a notch.''
John's friends are also impressed with Katy - who was previously married to Russell Brand - and believe she has changed him for the better.
The insider added to America's OK! magazine: ''John has changed. There's a kind of humility about him these days and that's due to Katy. He's said she's got the him, hook, line and sinker and he wants to spend the rest of his life making her happy.''
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