It was sort of funny, when I was younger I was much more Ti-rigid about relationships, expecting people to engage rationally, mocking and shunning sentiment. I remember even articulating for the person i eventually married that I couldn't tell them I loved them because it would be an irrational comment so early in the relationship, and I took love far more seriously than just to say something that would reflect transient feelings.
(I was really the romantic, wasn't I? Gawd.)
I still take my relationships very seriously, but I accept people as who they are nowadays rather than judge them stringently by my criteria. I can appreciate people for their own strengths; it's just that I know what sort of people I can make a deep personal investment in and which ones I can like and accept but just can't feel much for on a romantic and personal level.
I have a lot of trouble just having a "fling" and enjoying things until they dissipate. I will either hold part of myself back or I just won't bother. My intuition tells me upfront whether something will work and/or how much it will cost me to have a chance, and so I nip things in the bud up front; I've been in enough hard relationships to know now what I am willing to invest and not invest. I finally am in a spot where I *could* choose to indulge in a casual fling if I desired, but it's still a very weird feeling to me, to view a relationship as so transient.
With LTRs, I really really take it seriously and have to trust the person. I have a very playful casual side to me on the surface that I wear as a mask, to disarm people and keep them from reacting in extreme ways (I keep things low-key), and I suppose it's not really a "mask" because it is a valid facet of who I am; but inside I can get very serious and touchy especially with boundary issues. I want to go to the core of someone's being and have them to the core of mine, I want to be entirely emotionally intimate as human beings... but I can smell control issues a mile away and shy away from anyone who I feel is trying to manipulate or control me.
So it is this weird mix of playfulness on the surface but serious and passionate intensity underneath it all, it's hard to find people who can do both, who can go deep fearlessly without trying to control me to protect themselves.