Like the others, I collect with my heart, although I like to think my head makes an appearance when it's time to make a decision on cost/value! Occasionally I sell a gun at a loss, although most of the time I break even or make a small profit - which is then generally ploughed straight into 'investing' in another collectable.
One thing I've learned about myself over the years is how fickle I am, in terms of not being at all interested in something one minute and desperately yearning for it the next. Another is that I can talk myself into anything, such as 'if I buy this, I can sell the [less good] one I already have for £x, so I'm hardly really spending anything on it and it's much better than the one I have'. This calculation is usually made in about two milliseconds - the time it takes to click on the 'bid now' button.
PS I have a policy of never regretting a decision I've made...