If you look back on the series,you can see a different stage in the progression of Booth and Brennan's relationship which every two seasons covers.
Season 1-2 covers the stage of them being partners and friends but with an undercurrent of something more simmering under the surface which they can't admit or acknowledge.
Season 3-4 covers the stage of the intimate current between them continuing but now they have grown from being firmly in denial of it to poking around its edges without fully addressing what is between them like Booth and Brennan kissing under the mistletoe and trying to pretend it was nothing, Brennan asking Booth to be her sperm donor,and an increase in the gestures they make towards each other that seem small but have significant emotional value.
Season 5-6covers the stage ofBooth and Brennan's suppressed romantic feelings for each other being brought closer to the surface than ever before but instead of them admitting and expressing their feelings at the same time, they become two ships in the dark who always seem to miss each other. Booth confesses his feelings for Brennan first only to be rejected by Brennan forcing Booth to find some way of moving on from that. Then Brennan confesses her feelings for Booth but at a time when Booth is involved with someone else. But eventually, during the aftermath of Booth's breakup with his girlfriend, Booth and Brennan come together in the realization that they have these feelings for each other and they aren't going away. Them having sex and the resulting pregnancy were the final blows needed to tear down the emotional barrier between them that kept them from being together in the past.
Now Season 7 and the possible Season 8 will cover the stage of them being an official couple and parents to their unborn child,and the impactthose two thingshave on them and their lives.