We have worked on many opinion writing assignments this year. Students are familiar with using box and bullets to organize their thoughts, along with using parallel structure. Students were shown an example, and it was discussed, that each bullet needs to be turned into a paragraph. You can't just copy your two bullets into your letter and think it is done. You need to elaborate and say more about each of your reasons, and in this case, you should use some of the information you gathered about your pet to support your reasons.
The following pictures are the examples I showed the kids. We used them to discuss what their letters should look like- four paragraphs with elaboration. Many kids did just that, but some letters were so sparse because of lack of listening or lack of effort.