Because the color 'red' symbolizes many things - love, hate, passion, fury, vengeance, etc. Over the past 2 years, Taylor has experienced '0 to 100 relationships', she said so herself, which she now put into song (obviously) and the album title is appropriate for its theme.
She talked about songs that are about other people as well. Where she was an observer. But everybody is surely gonna assume these are all her relationships.
