The icy effect is more on the outside for the general people (--> SO) who don't get through to SP/SX. That's why instinct compatibility puts SP/SX and SX/SO together, SX is so energetic and frankly quite obtrusive, it really brings the heat. And they melt through SP naturally, getting to the secondary SX or SP/SX with ease.
SP and SP go well together, both are extremely stable. The most durable relationships are usually between SP/SX and SP/SO. Your certainly plays a role as well, your affections don't just change in a volatile SX or SO way. So, it needs a secure bank, and what is more secure than self-preservation?