Naomi Long has said she remains unclear as to why Lagan Valley MLA Trevor Lunn quit the Alliance Party but dismissed speculation that he felt he was being pushed aside in favour of Sorcha Eastwood.
Mr Lunn said on Monday that he had left the party over an "irreconcilable internal problem" which did not involve policy. He has been an MLA for 13 years and will now sit as an independent at Stormont.
Speaking for the first time about his resignation, the Alliance leader said she was perplexed as to why he had left but still considered him a friend.
"People want to find a reason. I'm sort of curious too," she said.
"I don't fully understand why it happened, but it happened and that's where we're at.
"I'm sad about it because, apart from anything else, I don't just consider Trevor a colleague. He was a friend. Trevor is still going to be a friend. It won't change relationships.
"It was his decision and I respect that. It won't spoil a friendship that stretches back years.
"It's disappointing, but we've a strong (party) association in Lagan Valley. We had our best ever Westminster result in December. We will be building on that and looking forward."
The party tripled its vote in the constituency in the election, with Ms Eastwood runner-up to the DUP's Sir Jeffrey Donaldson. Mrs Long said those suggesting that Mr Lunn was pushed aside in favour of her were "stirring the pot and going down the wrong track completely".
She added: "That does Trevor a disservice. He's been a big supporter of Sorcha. He's encouraged her in her political career. On polling day, he drove her around Lagan Valley. There's no animosity and no bad feeling between Trevor and Sorcha."
Mrs Long said that, like Mr Lunn, she did not want "to engage in recriminations and a slanging match" over his resignation. "People are always looking for a big drama. There's no big drama here," she added.
She said Mr Lunn had previously indicated he would not run again for the Assembly. She did not know if he had changed his mind since resigning but she was confident that, even if he did stand, Alliance would "retain the seat and potentially take two" in the constituency.