A love letter should always feel as personal as possible, states Hallmark. When writing to one's boyfriend, include things such as a favorite memory with that person, how one's boyfriend makes her feel and what the purpose of the letter is, whether it be a gift or just a simple reminder of his importance.
Within the letter, describe the little things he does that make the relationship work. Outline the specific reasons that person is appreciated without getting too mushy or cheesy. Note whether the person receiving the letter gets uncomfortable being too emotional or enjoys declarations of love and adjust the letter accordingly.
When writing a love letter, have a goal in mind, whether the goal is to bring tears of joy to the recipient's eyes or to make his birthday the best he has ever had, notes the Good Men Project. Include lines that incorporate the goal in mind. Begin the letter stating why this person deserves a love letter. Outline what exactly he brings to the table within the relationship, whether it be the smiles he gives or the support he shows. Be as detailed as he would feel comfortable with, because taking the time to write out feelings typically means more than just a simple "I love you."