Oscar Pistorius said text messages from Reeva Steenkamp expressing worries about their relationship were due to him being "jealous".
The Olympian and Paralmypian runner admitted feeling "insecure" about her talking to a man at a party the couple attended together, a week before he gunned her down.
Pistorius, 27, described being under pressure at the time over training, money and media scrutiny.
He and Steenkamp argued at an engagement party for his pal Darren Fresco. Afterwards, she sent him messages on WhatsApp complaining she felt bullied by him.
The texts were read out earlier in the Bladerunner's murder trial during the prosecution's case as evidence the couple had a troubled relationship.
Giving evidence for a second day at North Gauteng High Court, Pistorius said: "I was dealing with a lot of things: from the media side, from my training staff and finances. I was purchasing a new home and there were a lot of external factors."
Speaking of the row, Pistorius said he got angry because he wanted to leave the party ahead of a gruelling training session the following day, while Steenkamp delayed leaving by talking with a male friend.
Pistorius's anger was stoked further by a perceived slight after she failed to introduce him to her friend when he approached them.
He said: "At a point I wanted to get food as I was on a very strict diet, so we said we'd leave by a certain time. Near the end of the engagement, Reeva started talking to a gentleman unknown to me. She didn't introduce me to the person.
"I felt a bit upset. I was being sensitive, insecure and jealous. She started tickling my neck with her hand and I wasn't kind to her like I should've been. I said we had to leave."
An argument then broke out between them in the car park of the venue. He said: "She was in a short mood with me, so I dropped her off at her place and I didn't talk to her for the rest of the afternoon until she sent that message."
He said Steenkamp was an "emotional person" who was scarred by a previous difficult relationship which meant "she got her back up easily". He told the court he had not replied because he believed text messaging was an inadequate way of expressing emotions.
Pistorious claimed the incident had actually benefitted their relationship by showing her the demands of his lifestyle - in contrast to the meaning identified by the prosecution.
He said: "It's not easy for someone who hasn't dated a sportsman to understand how strict our diet and way of living is. I think [after] the argument at Darren's party she really did get the way I have to live my life."
The runner denies murder and claims he mistook Steenkamp for an intruder inside his luxury home on the outskirts of Pretoria. The prosecution alleges he killed her following an argument between the couple.
The trial continues.