Thoughts and feelings that arise automatically just are what they are. What matters is what new thoughts, feelings, words, and deeds we choose to make or not make.
As I understand, a fruit or result of karma (Pali: phala) is any experience that arises automatically in a person. For example, if someone insults or attacks you, the fear, anger, etc. that most people automatically feel is a result of past karmic seeds (Pali: bija) you have planted in your mind by choosing to feel that way under similar circumstances. Karma is intention (or mental energy), like setting or preference or default about how your mind should react in the future. Buddhists think that every automatic human experience (sights, sounds, moods, cravings, habits, cognition, etc.) is a karmic fruit — complex results from many past choices. Unless one practices making intentions more consciously, it is very easy for the fruits of past karma to run one’s life, and to keep perpetuating themselves if you keep giving them mental energy. It’s like a song stuck in your head that never stops as long as you keep singing it.
So, in terms of personal development, one has two good options: either try to have a happier future by learning to create happier, healthier, etc. karmic seeds (i.e., the path to a pleasurable, heavenly life), or learn not to make new karmic seeds and to destroy the seeds that already exist in the mind (i.e., the path to a peaceful, nirvanic life). For instance, for a more heavenly life, if a person attacks you, practice feeling pity, sympathy, or empathy for them instead of fear or anger, try to understand why they are attacking you and how you can reach some compromise with them or how you can help them out of whatever problem is prompting them to attack you. The brahmaviharas/divine-attitudes (metta/loving-kindness, karuna/compassion, mudita/sympathetic-joy, and upekkha/equanimity) are supposedly the mental states of heavenly beings. For a more nirvanic life, if a person attacks you, practice remaining calm and cool-headed, and make no responses, either mentally or physically, like a statue — even if a statue is destroyed by an attack, it does not respond. Nirvanic beings supposedly do not make any actions in this world, no matter what happens here. Here is an example of a (comedic, over-the-top) nirvanic response from a Vietnamese Buddhist man in the film Good Morning, Vietnam. The context is that Robin Williams’ character was teaching a funny English class about cursing.
“[Adrian (Robin Williams):] Let’s try a very special situation.
Wilkie, somethin’ special, okay?
You go into a restaurant okay?
A waitress comes up to you. You’re, eh–
You’re wearing your best new suit.
She comes up, she spills soup
all over you, looks at you like…
“Eh, I’m sorry.
What are you gonna do about it,
asshole?” What do you say to her?
What would you say? They spilled
something on your pants. What would
they do? What would you do?
[Wilkie (the Vietnamese man):] I do nothing.
[Adrian:] Come on, Wilkie. It’s cursing class.
You’re gettin’ a little pissed off.
What would you do?
[Wilkie:] I just remain reticent.
[Adrian:] Okay, she goes in the kitchen, she gets
a knife, she starts stabbing you.
She’s stabbing you.
She’s putting forks in you.
She’s got spoons in your eyes, Wil.
They’re startin’ to cut you with knives.
They’re puttin’ spoons in your eyes.
What would you do, Wil?
– What would you do?
[Wilkie:] I’m waiting to die! [everyone laughs]”
(transcript from Script-o-rama).