All unicorns have an “element” that bonds to their magic. This is seen in Earth Ponies and Pegasi too.
 
Common Earth Pony affinities include baking, growing food, and connections to nature. These aren’t necessarily limited to Earth Ponies, though, one example being Fluttershy, a pegasus with a nature affinity.
 
Common Pegasi affinities are usually associated with weather, like wind, thunder, or lightning. Also not limited to Pegasi, though. Tempest Shadow has lightning-based magic, which can also be referred to as electric-based magic. It’s common for children to inherit one of their parent’s affinities, and Cassie, Tempest’s daughter, actually inherited this lightning-based magic type. Most of the spells she perform come with bolts of electricity.
 
It is rare but possible for a pony to obtain two elemental alignments. Rainbow Dash, for example, has two: rainbows and sonic booms. A pony’s alignment doesn’t always have to come from their parents, some can be inherited from deep within a family’s bloodline.
 
 
Love Letter has fire-based magic. It is unknown where she inherited this trait from, as fire magic is extremely rare. It is more commonly observed among dragons, and is often associated with them because of this, like weather alignments are associated with Pegasi. Not all dragons have fire magic, though- Steven Magnet is a “sea” dragon who has water magic. There are earth and air dragons out there somewhere. Ember’s dragon pack mostly consists of fire-breathers, but she has some earth dragons in there too.
 
Love may have inherited her fire magic from Cadence’s bloodline, but since Cadence never knew her immediate family, there’s no way for Love to know where exactly she got it from. She has blue fire, so it’s very hot in temperature, and very dangerous to control. She first finds out about it in her early teen years, maybe when she was 13 or 14. Fire magic can be incredibly dangerous if not controlled and taught at a young age, but thankfully she was enrolled in classes with the most talented magic-users to gain control over her own magic. She even sought council from dragons. Over the years, she refines her magic to perfection.
 
The only other pony in recorded history to have possessed fire-based magic is Celestia herself. Celestia’s fire magic even extends to celestial bodies, like the sun and stars, hence her cutie mark and her role as the Queen of Equestria. Love Letter’s magic doesn’t extend this far, but as a fire user she does share an oddly close bond with the dragons of Equestria.
 
Over time, her magic becomes so concentrated and perfected that she doesn’t even need to verbally cast a spell for fire to appear. She can create it spontaneously, just by thought, which means her horn does not light up when she uses her fire magic. She does this intentionally. If she could start a controlled fire without evidence it was her then… why wouldn’t she? But it’s not like she’s randomly committing arson just because. It’s a skill she’s polished for style, and just in case she ever needs to use it. It could be useful.
 
It’s very hard, as a unicorn, to perform any sort of magic without having their horn light up. Even Twilight, the princess of magic, still has an aura around her horn when she uses magic, but it’s not necessarily on the top of her to-do list to get rid of the glow. She’s probably never thought too hard about practicing magic without the aura. If anything, it just makes magic harder.
 
Spontaneous creation is a very feared trait (take Discord for example).