Jealousy is a funny one for me. I'm not sure that I actually believe in it. I think jealousy is an umbrella term for whatever the root emotion is (insecurity, usually).
What R2F said about time was incredibly insightful, because time encompasses all those little things that can trigger the jealousy umbrella. A lack of time means that a person can feel, unwanted, rejected, unimportant, envious, etc.
I have found that different things trigger different emotions.
Perhaps I am not yet evolved enough - I would love to be at a stage where I thought "my partner and I are solid; therefore I have no insecurity."
For me, to date, this is not possible.
I trust in my partner and certainly, when we feel strong and in love, my feelings of insecurity almost completely dwindle.
However, I don't think that anything is certain in life. I don't think it's possible to say "I will always love you and be with you". I think it's possible to say "At the moment, I truly believe that I will always love you and be with you". For me, there's a slight difference.
Onto triggers for me...
Envy... this is one that occurs the least for me. When it happens, it can be because my partner is having a great time dating other people when I am not. How do I deal with that? Start hotting up my own dating life
... or simply let it pass if I choose not to date others. Envy of time has also been rare for me. I experienced it for the first time a couple of months ago. I'd been living with my GF and her husband for 3 months in the US. I came back to England in May. She told me that her and husband had a date and had sex a couple of days after I left. I felt my very first moment of envy in over a year; wishing it was me that was still there. I basically let it pass.
Insecurity... this is a more common one, when negative feelings do arise. These days, I am less fearful of my partner leaving me and more fearful of her *attention* leaving our relationship; so that I am stuck in an unhappy situation. This type of 'jealousy' can occur for me when she is in NRE - if she's talking about them a lot, spending a lot of time with them, tells me how hot they are, etc. I deal with it by letting it pass and also talking to her. I always find that talking to her really helps - especially if I precede it by joking "I know this is ridiculous, but I'm feeling a bit of this... can I get it off my chest?"
Possessiveness... my girlfriend jokes that she's possessive and I'm jealous (insecure). She says that her knee-jerk response to me kissing someone is "hey, that's my sexy girlfriend!", whereas mine might be "I wonder if they kiss better than I do?"
I've noticed that I have started to feel more 'possessive' and less 'insecure' recently. She slept with her first person outside of our 1.5yr relationship a couple of days ago and my main response was mild envious possessiveness. A feeling of "he gets to touch her and I don't".
The final one, which is actually the most predominant of all of the 'jealousy' umbrella emotions for me, is anger/unhappiness at being mistreated. I want to feel important to my partner. I want to feel considered. If she does something like making a date with a new person and merely informing me of it, instead of saying "they've asked me out, would you feel ok if I went out with them?", I feel somewhat unimportant in the grander scheme of her life.
Whatever the root cause, yes, I do feel those pangs of the 'jealousy' umbrella. The pangs can be quite strong, or very small, depending on the situation.
All of this being said - these emotions don't consume me or our relationship. I have them, but they are not an every second of the day thing
Your cake analogy is very interesting! In terms of that, I'd say that as long as my slice of cake is what I consider to be satisfying enough, then I am happy to share the rest of the cake.