It wasn't a love potion. Lavender had a crush on Ron, and when they went out, he was just so excited to have someone actually like him like that, so he just went with it. But Ron got sick of her in like the first week.
Well I don't think he had accepted his crush on Mione just yet, he was a teenage boy and it wasn't really love. I think in the beginning he might have crushed on Lavender too a bit (when he realised she liked him, so it was more the attention he got) but overall I think he sort of slipped into this relationship and saw no reason to break it off until he did.
