Let's start with this the Tempo is like the metronome of the song, you either want it to be a slow, mid pase or fast song.

Most of the songs I write I already hear the tempo the rhythm in my mind.

If it's a slow song it'll be more romantic with wording also depends on HOW you word it as well. Words can either make the song fast or slow.

Think about the lyrics carefully and do they really go with this tempo?

The tempo is the KEY ingredient to making a song