Just to give a concrete example, for instance I don't understand why belief in strong national defense has to go hand in hand with being anti abortion and anti gay marriage.
They don't. For example, Joe Lieberman is strong on national defense and liberal on issues like abortion and same sex marriage. Part of the confusion here is that in the United States, every legislator is forced to align (or caucus) with one of the two parties. Even Angus King of Maine caucuses with the Democrats despite being nominally independent.
In many other countries, parties are smaller and more specific. Voters pick the exact party that they want and then the politicians work out the coalitions after that. This produces similar coalitions in effect, as a strong national defense party might ally with a party that is against abortion and same sex marriage. But it's clearer that the coalitions are temporary. And because politicians make the coalitions and not voters, they are easier to dissolve. Voters aren't invested in them.
In the US, voters pick the president (somewhat indirectly but still solely; their presidential vote doesn't have to determine their votes for legislatures unless they choose to do so). Voters may be picking their candidate based on national defense, but they get a candidate with a multitude of positions. Supporters don't necessarily need to support all the positions, but they need to be able to make peace with them.
Abortion in particular is an example of an area where things have changed. In the 1970s, anti-abortion (pro-life) was a Democrat issue. Southern Democrats like Bill Clinton and Al Gore and Catholic Democrats like Bill Coyne were officially pro-life. Also, at that time, pro-life meant both anti-abortion and against the death penalty.
In 1980, Reagan changed that. He ran on an explicitly pro-life platform. Due to his success, most everyone in the Republican party is at least nominally pro-life and all but a few Democrats are pro-choice (pro-abortion availability). Even those who are nominally pro-life (like Bob Casey, Jr.) vote pro-choice on the few active issues (Supreme Court nominations and support for Planned Parenthood).
The abortion and death penalty issues separated. Most evangelicals are anti-abortion and for the death penalty, invoking separate principles for fetuses and criminals. Others had previously linked them as both human beings. Both are logical positions, but the logic of course follows a different path.
One reason for a politician to pick up certain issues is because they are strongly held. For example, in 1980, roughly 8% of voters said they voted only on abortion. 2% for and 6% against. So Reagan's position had the effect (if not the intent) of giving him a net 4% gain in voters. And it was part of a larger stance, a moral stance. Reagan ran as the champion of moral uprightness (against premarital and extramarital sex, abortion, etc. and for marriage). That was considered one leg of his platform. The other two were strong defense and low taxes. Where low taxes could be generalized out to include low domestic spending and limited regulation. In combination those gave him a political majority.
Notice how low spending and a strong national defense are in conflict. He chose strong defense over low spending (and strong defense over low debt). Someone else might prefer low spending. Someone else might put domestic and defense spending in conflict and favor one over the other. For example, a common Democrat position is that spending cuts in defense should be used to pay for more social programs. Lieberman favors high taxes and a strong national defense (and high domestic spending).
The joke in the 1980s was that voters selected a Democratic Congress to get lots of goodies and a Republican president so they wouldn't have to pay for it.
Politics is often a matter of compromise. In parliamentary systems, that compromise is often made by politicians after elections, when they form coalitions to choose a prime minister. In presidential systems (where the president is chosen separately from legislators), that compromise is often made by voters. Historically voters would hedge their votes a bit, voting for a president from one party and a Senator from the other. But in the 2016 election, they didn't do that in any state.
In your particular case, you might find yourself becoming more partisan if you were more interested in politics. Because people have been increasingly picking sides. You've stayed out of it, so your beliefs can be more free form and don't need to conform to the political choices that you are making.